diff --git a/image_analysis/scale_merge/RotationScaleMerge.cpp b/image_analysis/scale_merge/RotationScaleMerge.cpp
index 3a53277c2..a644b54a9 100644
--- a/image_analysis/scale_merge/RotationScaleMerge.cpp
+++ b/image_analysis/scale_merge/RotationScaleMerge.cpp
@@ -5439,16 +5439,29 @@ RotationScaleMerge::Result RotationScaleMerge::Run(bool for_search, bool full_st
     }
 #endif
     if (!scaled_on_gpu) {
+        // RunScalingLoop rescales after every iteration before anything reads corr, so the iteration's
+        // UpdateCorr is folded into that pass over the partials: the G and the fitted frames as the
+        // iteration left them, then the rescale on top - the same two roundings, one pass instead of two.
+        std::vector<double> g_fitted;
+        std::vector<uint8_t> frame_fitted;
         partial_loop = RunScalingLoop("partials", g_partial, frame_count, smooth_window,
                                       /*release_restraint=*/false, [&] {
             ReducePartialGroupMeans(n_groups, partial_mean);
             FitPerFrameG(partials, frame_start, frame_count, partial_mean, /*unity=*/false, g_partial);
-            UpdateCorr(partials, g_partial, frame_scaled_scratch);
+            g_fitted = g_partial;
+            frame_fitted = frame_scaled_scratch;
         }, [&](const std::vector<uint8_t> &apply, const std::vector<double> &ratio) {
             ParallelChunks(static_cast<int>(partials.size()), ThreadsForWork(partials.size(), nthreads),
                            [&](int lo, int hi) {
                 for (int i = lo; i < hi; ++i) {
                     Obs &o = partials[i];
+                    if (frame_fitted[o.frame]) {   // UpdateCorr
+                        const double denom = static_cast<double>(o.partiality) * g_fitted[o.frame];
+                        if (std::isfinite(o.prescaling_corr) && std::isfinite(denom) && denom > 0.0)
+                            o.corr = static_cast<float>(o.prescaling_corr / denom);
+                        else
+                            o.corr = NAN;
+                    }
                     if (apply[o.frame] && std::isfinite(o.corr))
                         o.corr = static_cast<float>(o.corr * ratio[o.frame]);
                 }