diff --git a/docs/port/f6-what-starts-the-sweep.md b/docs/port/f6-what-starts-the-sweep.md
index c39109d7..f8d10e2e 100644
--- a/docs/port/f6-what-starts-the-sweep.md
+++ b/docs/port/f6-what-starts-the-sweep.md
@@ -57,6 +57,40 @@ with `pteff03` having already crossed the measured band and with the coarse
 resize washing a thin glow out. **The film neither adds to nor contradicts the
 declaration; the declaration is the evidence here.**
 
+## Where the sweep actually is, computed from the leaf's own translation
+
+The port positions the leaf **from the leaf's own clock** — it does not draw the
+parent's pose and ignore the translation. Sprite 399 wide on a 1280 screen:
+
+| t | 0 | **61** | 70 | 100 | 150 | **236** | 250 |
+|---|---|---|---|---|---|---|---|
+| `pteff03` x | −639 | **−395** | −359 | −239 | −39 | **305** | 361 |
+| `pteff03a` x | 1721 | 1477 | 1436 | 1314 | 1111 | 762 | 705 |
+
+**`pteff03` enters the frame at t=61 and is mid-screen at t=305 when the plate
+reaches full alpha at t=236** — visible and travelling for ~175 units ≈ 2.9 s
+before the plate. `pteff03a` enters much later.
+
+## 🔴 The open question in this file sits exactly inside F6's window
+
+`screen_view.gd` flags its own limit on the leaf-vs-parent alpha decode:
+
+> *"Every observation behind this has parent alpha 0, so 'the leaf wins' and 'the
+> parent is ignored because it draws nothing' are NOT separated. **A capture
+> during t=100…238 would separate them.**"*
+
+The parent is non-zero exactly on `t=100…238`, and the plate arrives at 236. So
+that unresolved ambiguity is **the same interval F6 is about**, and it is
+load-bearing for the first stretch: the parent ramps 70→100, so applying it would
+hide the sweep until t=70 and dim it to t=100, while the port shows it at full
+alpha from t=61.
+
+That accounts for ~40 units of the earliness. **It does not account for the other
+~175**, which is the leaf clock starting at title t=0 with no offset.
+
+📌 **One capture in `t=100…238` would settle both** — F6's start time and the
+leaf/parent alpha rule — rather than two.
+
 ## What changes when the answer lands
 
 A start time is an **offset**, and `leaf_time_units` is an absolute override —