diff --git a/image_analysis/scale_merge/RotationScaleMerge.cpp b/image_analysis/scale_merge/RotationScaleMerge.cpp
index 6470a8a0..680e0004 100644
--- a/image_analysis/scale_merge/RotationScaleMerge.cpp
+++ b/image_analysis/scale_merge/RotationScaleMerge.cpp
@@ -1869,8 +1869,13 @@ void RotationScaleMerge::ApplyCellSurface(const std::vector<int32_t> &cell, int
         std::fill(swI.begin(), swI.end(), 0.0);
         for (const int32_t i : sel) {
             const Obs &o = fulls[i];
-            const double a = A[cell[i]], sc = static_cast<double>(o.sigma) * o.corr * a, w = 1.0 / (sc * sc);
-            sw[o.group] += w; swI[o.group] += w * static_cast<double>(o.I) * o.corr * a;
+            // Keep the intensity's own product together before it is weighted: w * ((I*corr)*a) is
+            // what the loops this replaced summed, and w * I * corr * a rounds differently. The
+            // score decides whether a surface is kept, so a last-place difference here can flip a
+            // gain gate that is otherwise sitting on the fence.
+            const double a = A[cell[i]], Is = static_cast<double>(o.I) * o.corr * a;
+            const double sc = static_cast<double>(o.sigma) * o.corr * a, w = 1.0 / (sc * sc);
+            sw[o.group] += w; swI[o.group] += w * Is;
         }
     };