diff --git a/image_analysis/IndexAndRefine.cpp b/image_analysis/IndexAndRefine.cpp
index c1b0ac7e..89c938d7 100644
--- a/image_analysis/IndexAndRefine.cpp
+++ b/image_analysis/IndexAndRefine.cpp
@@ -292,7 +292,10 @@ void IndexAndRefine::QuickPredictAndIntegrate(DataMessage &msg,
     float mos_deg = 0.1f;
 
     if (experiment.GetGoniometer().has_value()) {
-        wedge_deg = experiment.GetGoniometer()->GetWedge_deg() / 2.0;
+        // Full oscillation wedge of one frame; BraggPredictionRot halves it to the +/- half-wedge of the
+        // partiality erf pair (Kabsch). Passing the full increment gives a half-wedge of increment/2 -
+        // matching ScaleOnTheFly's RotationPartiality, so the predicted partiality is used directly there.
+        wedge_deg = experiment.GetGoniometer()->GetWedge_deg();
 
         if (msg.mosaicity_deg) {
             mos_deg = msg.mosaicity_deg.value();
diff --git a/image_analysis/scale_merge/ScaleOnTheFly.cpp b/image_analysis/scale_merge/ScaleOnTheFly.cpp
index fe909feb..086fc928 100644
--- a/image_analysis/scale_merge/ScaleOnTheFly.cpp
+++ b/image_analysis/scale_merge/ScaleOnTheFly.cpp
@@ -269,6 +269,12 @@ void ScaleOnTheFly::Scale(IntegrationOutcome &integration_outcome) const {
         result.wedge = NAN;
     }
 
+    // The partiality stored on each reflection was computed at prediction with this image's mosaicity and
+    // the (now correct) rotation wedge, so it is used directly. Recompute it only when scaling overrides
+    // that geometry: wedge refinement, a forced mosaicity, or a scaling wedge override.
+    const bool recompute_partiality = model == PartialityModel::Rotation
+        && (refine_rot_wedge || s.GetForcedMosaicity().has_value() || s.GetRotationWedgeForScaling().has_value());
+
     auto clear_scale = [&]() {
         integration_outcome.image_scale_cc.reset();
         integration_outcome.image_scale_cc_n.reset();
@@ -298,7 +304,9 @@ void ScaleOnTheFly::Scale(IntegrationOutcome &integration_outcome) const {
                     partiality = 1.0;
                     break;
                 case PartialityModel::Rotation:
-                    partiality = RotationPartiality(r.delta_phi_deg, r.zeta, result.mos, result.wedge);
+                    partiality = recompute_partiality
+                        ? RotationPartiality(r.delta_phi_deg, r.zeta, result.mos, result.wedge)
+                        : r.partiality;
                     break;
                 default:
                     partiality = r.partiality;
@@ -399,7 +407,8 @@ void ScaleOnTheFly::Scale(IntegrationOutcome &integration_outcome) const {
                 r.partiality = 1.0;
                 break;
             case PartialityModel::Rotation: {
-                if (std::isfinite(r.delta_phi_deg) && std::isfinite(r.zeta) && result.mos > 1e-6)
+                if (recompute_partiality && std::isfinite(r.delta_phi_deg) && std::isfinite(r.zeta)
+                    && result.mos > 1e-6)
                     r.partiality = RotationPartiality(r.delta_phi_deg, r.zeta, result.mos, result.wedge);
                 break;
             }