diff --git a/SURFACE_LEVEL_PLAN.md b/SURFACE_LEVEL_PLAN.md
index 19327db..9f87f01 100644
--- a/SURFACE_LEVEL_PLAN.md
+++ b/SURFACE_LEVEL_PLAN.md
@@ -1,6 +1,6 @@
 # Surface Level — Implementation Attack Plan
 
-**Status:** Proposed; send to Claude for audit before implementation.  
+**Status:** Audited; ready for implementation after this revision.
 **Scope:** Surface Level only. Do not change Angle, Edge, ruler, or billing in this stage.
 
 ## Outcome
@@ -28,8 +28,13 @@ The user should be able to answer three questions at a glance or by sound:
 - Surface means the phone is intended to lie flat, screen-up, on the plane being
   measured. Its level-lock behavior is valid only in that geometry.
 - Numeric measurements are never hidden because the phone is steeply oriented.
-  Outside the ideal Surface geometry, show the actual angle and a quiet placement
+  Surface tilt magnitude remains authoritative from 0 to 180 degrees. Outside
+  the ideal face-up geometry, show the actual magnitude and a quiet placement
   hint; suppress level-lock feedback rather than replacing the reading with `—`.
+- Above 80 degrees of tilt, de-emphasize/suppress Pitch and Roll because their
+  off-axis parameterization becomes noisy near vertical. Above 90 degrees,
+  identify the screen-down-side state rather than pretending the bullseye is a
+  useful placement tool. The magnitude remains visible in both cases.
 - The central visualization is a bullseye vial. It has a finite visual range:
   beyond that range the bubble rests at the rim, while numeric information remains
   full-range and authoritative.
@@ -42,20 +47,30 @@ The user should be able to answer three questions at a glance or by sound:
 
 - Refactor Surface placement gating so it only controls the level-lock state,
   lock haptic, and geometry-specific guidance.
-- Keep primary angle, pitch, and roll visible at 0-90 degrees.
-- Add tests at flat, near level, 30 degrees, and near vertical proving numeric
-  readings remain present and correct; verify invalid placement never locks.
+- Keep the primary tilt magnitude visible from 0-180 degrees. Keep Pitch and
+  Roll visible only where their meaning is stable (below 80 degrees tilt).
+- Add tests at flat, near level, 30 degrees, 80 degrees, near vertical, and
+  screen-down. Verify magnitude stays correct, off-axis values are suppressed
+  in their ambiguous range, and invalid placement never locks.
 - Keep the existing lock tolerance semantics: a lock label states the tolerance
   rather than pretending any non-zero value is exactly flat.
 
 ### 2. Add the guided Surface calibration flow
 
 - Add a Surface Calibration route from the Level header.
-- Explain that the phone may be calibrated on any firm, stable surface; the
-  surface itself does not need to be level.
-- Step 1: settle, sample for a short stable window, capture.
-- Step 2: rotate the phone exactly 180 degrees in the same plane, settle, capture.
-- Reject/retry if readings indicate motion or inconsistent placement.
+- Explain that the phone may be calibrated on any firm, stable, *roughly level*
+  surface; it need not be perfectly level. Warn and ask for a flatter surface
+  when the measured tilt exceeds 5 degrees.
+- Capture the two samples from the uncalibrated sensor path. Never derive a new
+  calibration from readings that an existing calibration has already corrected.
+- Step 1: settle, collect a 1.5-second stable window, capture.
+- Step 2: rotate the phone exactly 180 degrees in the same plane, settle, then
+  collect and capture another 1.5-second stable window.
+- Use the shared settling detector: enter Settling above 1.0 degrees/second of
+  two-axis plane movement; leave it only after movement is at or below 0.3
+  degrees/second for 500 ms. Reject/retry windows that are not settled.
+- Reject/retry a derived calibration bias above 3 degrees as evidence that the
+  device, setup, or turn was inconsistent. Suppress lock feedback during the flow.
 - Persist only the resulting Surface calibration; never alter Edge calibration.
 - State that the calibration applies to the current phone/case/contact setup.
 - On completion, show a concise success result and a way to clear/re-run it.
@@ -67,10 +82,19 @@ The user should be able to answer three questions at a glance or by sound:
 - Glass, fluid, and meniscus effects must improve depth/readability; avoid a
   decorative or excessively glossy treatment.
 - Bubble target position derives directly from stable calibrated pitch/roll.
-- Bubble movement uses a critically damped animation toward that real target:
-  no random drift, autonomous wobble, or animation that contradicts the number.
+- Derive bubble target, high/low instruction, and rise/run direction from one
+  pure SurfaceGuidance function. Its hysteretic next-state input must prevent
+  quadrant labels from flapping at an axis boundary. Use an edge label when the
+  minor axis is under 25% of the dominant axis; otherwise use a corner label.
+- Bubble movement uses a critically damped spring (damping ratio >= 1) toward
+  that real target: no overshoot, random drift, autonomous wobble, or animation
+  that contradicts the number. Clamp the target to the vial's visual range before
+  applying the spring so the bubble settles at the rim.
+- Etched rings are real visual calibration marks, not decoration: map them to
+  explicit degree values (initially 1, 2, and 5 degrees) using the same pure
+  measurement-to-position mapping as the bubble.
 - Respect disabled system animations and the app's reduced-motion preference by
-  minimizing or removing the display spring.
+  direct-positioning the deadbanded bubble value instead of running the spring.
 - Show a large total tilt beside/below the vial, then Pitch and Roll below it.
 - At level lock, use the restrained lime state and an explicit text label.
 
@@ -82,32 +106,42 @@ The user should be able to answer three questions at a glance or by sound:
   millimetres per metre and/or inches per foot based on the selected unit system.
 - Treat exact board/counter dimensions and calculated shim/foot height as a later
   Adjustment Assist feature, not a requirement of this stage.
-- When the reading is moving, show a subtle `Settling` state and defer strong
-  correction guidance until the stable measurement is trustworthy.
+- Use the shared settling detector from calibration. When it is settling, show a
+  subtle `Settling` state and defer strong correction guidance until the stable
+  measurement is trustworthy.
+- Add a persisted units preference in core/settings for rise/run formatting;
+  the Tools settings UI remains deferred.
 
 ### 5. Add optional hands-free Audio Level Assist
 
 - Surface-only, off by default, explicitly user-enabled.
 - Use a soft sonar-like pulse with a gentle attack and naturally fading tail;
   never a harsh alarm or rapid smoke-detector cadence.
-- Silent while settling or materially outside the usable placement geometry.
-- Near level: occasional low-key proximity pulses.
+- Silent while the shared settling detector is active or materially outside the
+  usable placement geometry.
+- Near level: occasional low-key proximity pulses with hysteresis between audio
+  proximity bands, so band changes cannot chatter.
 - Locked: one distinct soft lock ping; optionally provide a sparse repeat-while-
   locked preference so a user adjusting a large plank can hear that it remains level.
 - Reuse the existing lock dwell/hysteresis/debounce so sound cannot chatter.
-- Respect media volume, audio focus, system sound settings, app lifecycle, and
-  the user's Audio Assist preference.
+- Use SoundPool with `USAGE_ASSISTANCE_SONIFICATION`; mix with media rather than
+  taking audio focus for intermittent pulses. Respect media volume, system sound
+  settings, app lifecycle, and the user's Audio Assist preference. Release audio
+  resources with the tool composable/flow collection.
 
 ## Surface acceptance criteria
 
 - A stable phone on a known level plane achieves lock without repeated feedback.
-- A 180-degree calibration on a stable but non-level plane improves repeatability
-  without zeroing that plane's real slope.
-- A screen-up phone at 30 and near 90 degrees still shows meaningful numbers;
-  it does not falsely claim Surface lock.
-- Bubble direction agrees with Pitch/Roll and with the high/low instruction.
-- Bubble visual remains physically believable but never leads or disagrees with
-  the stable numeric reading.
+- A 180-degree calibration from uncalibrated stable samples on a stable,
+  roughly-level plane improves repeatability without zeroing that plane's real
+  slope; inconsistent/moving captures are rejected.
+- A screen-up phone at 30, 80, near 90, and screen-down angles always shows a
+  correct magnitude. Pitch/Roll are suppressed at their documented ambiguous
+  range, and the Surface tool never falsely claims a lock.
+- Bubble direction, high/low instruction, and rise/run direction agree in all
+  four quadrants through one pure, unit-tested guidance function.
+- Bubble position is a pure mapping of stable measurement plus a no-overshoot
+  spring. Its etched rings match that mapping's documented degree marks.
 - Audio can guide a user to lock without viewing the screen and stays quiet when
   disabled, settling, backgrounded, or outside the Surface geometry.
 - Manual checks cover phone-with-case, phone-without-case, a firm tabletop, and