diff --git a/docs/re/structures/ui-resting-pose.md b/docs/re/structures/ui-resting-pose.md
index 14155eb0..b825d9a0 100644
--- a/docs/re/structures/ui-resting-pose.md
+++ b/docs/re/structures/ui-resting-pose.md
@@ -120,6 +120,40 @@ moved 0.001, against an in-box capture noise of 0.32. A margin between two rende
 scored on one capture is far more robust than either score is — which is why the
 render axis, not the capture axis, is the binding constraint here.
 
+### ✅ The capture-phase term, measured — and why the box was the right choice
+
+`sylpheed-port` overturned their own phase-0 result using the identical-leaves
+fact: sweeping the *same* leaf against a `title` capture minimises at phase 240
+where `main_menu` minimises at 0, so **the best-matching phase is a property of
+when the shutter fell, not of the game's rest state**. A continuously sweeping
+element has no canonical rest phase. They then warned that any whole-frame score
+against a single capture carries an irreducible phase term of ~1.0 RMSE.
+
+That term is measurable on my own two JP sessions, which certainly differed in
+sweep phase — 44 025 px differ in the band the leaf crosses:
+
+| region scored | between-session RMSE |
+|---|---|
+| **whole frame** | **4.566** |
+| the sweep band, x 721..1241 | 4.088 |
+| **the adjudication box** | **0.3215** |
+
+⚠️ **Their ~1.0 understates it for this screen.** A whole-frame score against one
+capture of the JP title carries **~4.6**, not ~1.0 — theirs is the leaf-phase
+component isolated in a renderer, mine is everything that varies between sessions
+(the plate pulse alone contributes ~2.8, measured separately on the EN pair) and
+includes theirs. Anyone quoting a whole-frame number on this screen should use the
+larger figure.
+
+📌 **And it is why the box matters more than I realised when I chose it.** Scoring
+the 388×423 region rather than the frame drops the between-session term from 4.566
+to **0.3215** — a factor of 14 — because the sweep contributes at x 721..1241 and
+the box is mostly clear of it. That was not the reason I cropped (the crop was to
+avoid diluting a local difference across 92 % of an identical frame), so the
+robustness is luck. **The general rule it earns: score inside a region that
+excludes the free-running elements, and *measure* the residual term there rather
+than estimating it.**
+
 **So the margins re-stated against every noise floor now measured:**
 
 | comparison | margin | render noise (1.2) | in-box capture noise (0.32) |