diff --git a/image_analysis/scale_merge/RotationScaleMerge.cpp b/image_analysis/scale_merge/RotationScaleMerge.cpp
index a644b54a9..ae82fcf68 100644
--- a/image_analysis/scale_merge/RotationScaleMerge.cpp
+++ b/image_analysis/scale_merge/RotationScaleMerge.cpp
@@ -510,6 +510,7 @@ void RotationScaleMerge::Ingest() {
     // straight from the source reflections (and the smoothing between there and here never
     // touches corr).
     if (!resident_ingest) {
+        rocking_event_frames_at_start = -1;
         corr_ingested.resize(partials.size());
         ParallelChunks(static_cast<int>(partials.size()), ThreadsForWork(partials.size(), nthreads),
                        [&](int lo, int hi) {
@@ -5408,12 +5409,14 @@ RotationScaleMerge::Result RotationScaleMerge::Run(bool for_search, bool full_st
     const double osc_deg = gonio ? std::fabs(gonio->GetIncrement_deg()) : 0.0;
     int smooth_window = 1;
     if (smooth_g_deg > 0.0 && osc_deg > 1e-6) {
+        if (rocking_event_frames_at_start < 0)
+            rocking_event_frames_at_start = RockingEventFrames();
         smooth_window = std::max(static_cast<int>(std::lround(smooth_g_deg / osc_deg)),
-                                 SMOOTH_G_MIN_ROCKING_EVENTS * RockingEventFrames());
+                                 SMOOTH_G_MIN_ROCKING_EVENTS * rocking_event_frames_at_start);
         if (smooth_window % 2 == 0) ++smooth_window;
         logger.Info("Per-frame scale smoothing window: {} frames ({:.1f} deg; {} deg asked, rocking "
                     "curves span {} frames)", smooth_window, smooth_window * osc_deg, smooth_g_deg,
-                    RockingEventFrames());
+                    rocking_event_frames_at_start);
     }
     ScalingLoopOutcome partial_loop;
 #ifdef JFJOCH_USE_CUDA
diff --git a/image_analysis/scale_merge/RotationScaleMerge.h b/image_analysis/scale_merge/RotationScaleMerge.h
index 4db567d58..f0806d8b4 100644
--- a/image_analysis/scale_merge/RotationScaleMerge.h
+++ b/image_analysis/scale_merge/RotationScaleMerge.h
@@ -288,6 +288,9 @@ private:
     // nine-iteration merge is wrong. A later pass inherits that state rather than creating it, so this
     // is a defect of the P1 search arm, and it reaches the user through the point group that arm picks.
     std::vector<float> corr_ingested;
+    // RockingEventFrames at the start of a Run, where corr has just been restored to corr_ingested - so
+    // the same for every Run on the same ingest; -1 until the first Run takes it.
+    int rocking_event_frames_at_start = -1;
 
     // Raw-hkl ordering, built ONCE by Ingest and reused: `perm` lists partial indices sorted by
     // (raw h,k,l, image_number); each distinct raw hkl is a contiguous run [rawrun_start, +count) of it.