diff --git a/image_analysis/IndexAndRefine.cpp b/image_analysis/IndexAndRefine.cpp
index 2ec7c4e0..c6455113 100644
--- a/image_analysis/IndexAndRefine.cpp
+++ b/image_analysis/IndexAndRefine.cpp
@@ -457,6 +457,13 @@ void IndexAndRefine::QuickPredictAndIntegrate(DataMessage &msg,
     // report that default onward: RotationScaleMerge averages the reported values in frame order to
     // recompute every partiality, and a placeholder entered there is read as a measurement.
     std::optional<float> mos_measured;
+    // The width the PREDICTION window opens to, which is a different question from the width the
+    // partiality divides by. Prediction only decides membership, and a reflection just inside a
+    // generous window arrives with a partiality near zero and is weighted accordingly, so erring wide
+    // there is close to free (measured: a 28% wider window moves the merged statistics by under half a
+    // percent). The divisor is not free - it multiplies every partial - so it must stay on the width
+    // this frame actually measured.
+    std::optional<float> mos_predict;
 
     if (experiment.GetGoniometer().has_value()) {
         // Full oscillation wedge of one frame; BraggPredictionRot halves it to the +/- half-wedge of the
@@ -474,15 +481,10 @@ void IndexAndRefine::QuickPredictAndIntegrate(DataMessage &msg,
         // reflections and collapse the multiplicity. (A wider value only helps prediction cover the spot.)
         if (msg.number >= 0 && msg.number < static_cast<int64_t>(prediction_mosaicity_override_.size())
             && std::isfinite(prediction_mosaicity_override_[msg.number])
-            && prediction_mosaicity_override_[msg.number] > 0.0f) {
-            mos_measured = std::max(mos_measured.value_or(0.0f), prediction_mosaicity_override_[msg.number]);
-            mosaicity[msg.number] = *mos_measured;
-        }
+            && prediction_mosaicity_override_[msg.number] > 0.0f)
+            mos_predict = std::max(mos_measured.value_or(0.0f), prediction_mosaicity_override_[msg.number]);
     }
-    // The width the prediction window opens to. It is the measured sigma_M unless a fixed prediction
-    // mosaicity was asked for: the reported (and therefore integrated-against) sigma_M below is left
-    // alone either way, so the two uses of the number can be separated.
-    float mos_deg = mos_measured.value_or(0.1f);
+    float mos_deg = mos_predict.value_or(mos_measured.value_or(0.1f));
     if (const auto forced_pred = experiment.GetBraggIntegrationSettings().GetForcedPredictionMosaicity_deg())
         mos_deg = *forced_pred;
 
diff --git a/rugnux/rugnux_cli.cpp b/rugnux/rugnux_cli.cpp
index ac2b012a..253da9a6 100644
--- a/rugnux/rugnux_cli.cpp
+++ b/rugnux/rugnux_cli.cpp
@@ -149,7 +149,7 @@ void print_usage() {
     std::cout << "   --capture-uncertainty <num>     rot3d: systematic sigma ~num*(1-captured_fraction)*I on under-captured fulls (default: 1.0 for rot3d, 0 otherwise)" << std::endl;
     std::cout << "   --min-captured-fraction <num>   rot3d: drop a combined full whose rocking curve was captured below this fraction (edge-of-sweep truncated fulls) (default: 0.7 for rotation, 0 otherwise; 0 = off)" << std::endl;
     std::cout << "   --mosaicity <num>               Diagnostic: fix the scaling mosaicity (deg) instead of the per-image seed" << std::endl;
-   std::cout << "   --prediction-mosaicity <num>    Diagnostic: fix the rocking width the PREDICTION window opens to (deg), leaving the partiality on the per-image sigma_M. Prediction and partiality are one number by default, so a sigma_M that moves takes the integrated reflection population with it" << std::endl;
+    std::cout << "   --prediction-mosaicity <num>    Diagnostic: fix the rocking width the PREDICTION window opens to (deg), leaving the partiality on the per-image sigma_M. Prediction and partiality are one number by default, so a sigma_M that moves takes the integrated reflection population with it" << std::endl;
     std::cout << "   --reject-outliers <num>         Per-observation merge outlier rejection, N sigma from the per-reflection median (default: 6 for rot3d, XDS/DIALS-style; 0 = off)" << std::endl;
     std::cout << "   --search-min-zeta <num>         De-novo space-group search only: also search a merge of just the observations whose Lorentz geometry |zeta| reaches this, and keep whichever search found MORE symmetry (default: 0.85 for rotation, 0 = single search). Reflections crossing the Ewald sphere near-tangentially are measured worst and can make a real symmetry operator look like a twin law" << std::endl;
     std::cout << "   --min-image-cc <num>            Per-image CC limit in percent (default: no limit)" << std::endl;