diff --git a/image_analysis/scale_merge/RotationScaleMerge.cpp b/image_analysis/scale_merge/RotationScaleMerge.cpp
index 813782e4..35180df5 100644
--- a/image_analysis/scale_merge/RotationScaleMerge.cpp
+++ b/image_analysis/scale_merge/RotationScaleMerge.cpp
@@ -554,9 +554,10 @@ void RotationScaleMerge::RefineDecay(int n_groups) {
     // noise / aggregate structure (not real per-reflection decay) would amplify into a high-res
     // mis-correction and worsen the symmetry-equivalent scatter that sets ISa. An opt-in correction must
     // never add noise where its systematic is absent (a low-dose reference set), so the slope is
-    // CROSS-VALIDATED: fit on even-frame fulls and score the held-out odd-frame equivalent chi^2, and vice
-    // versa. Real decay generalizes across the split (both held-out scores improve); a noise slope does
-    // not. Apply the full-data slope only if the mean held-out chi^2 strictly improves; else it is a no-op.
+    // CROSS-VALIDATED: fit on even-frame fulls and score the held-out odd-frame equivalent agreement (a
+    // sigma-independent Rmeas-like metric), and vice versa. Real decay generalizes across the split (both
+    // held-out scores improve); a noise slope does not. Apply the full-data slope only if the mean held-out
+    // agreement strictly improves; else it is a no-op.
     if (fulls.empty() || n_frames <= 0)
         return;
     const double fcenter = 0.5 * n_frames;
@@ -593,9 +594,14 @@ void RotationScaleMerge::RefineDecay(int n_groups) {
         // strong damage (slope = 2 dB/dframe, so +-1.0 admits total relative-B up to ~n_frames/2 A^2).
         return (var > 0.0) ? std::clamp((Sxy - Sx * Sy / Sw) / var, -1.0, 1.0) : 0.0;
     };
-    // Mean studentized squared deviation over the subset {frame&1 == parity} when `slope` is applied,
-    // scored against that subset's own reference (no leakage). Lower = tighter equivalents.
-    auto subset_chi2 = [&](int parity, double slope) -> double {
+    // Rmeas-like (sigma-INDEPENDENT) disagreement of the equivalents over the subset {frame&1 == parity}
+    // when `slope` is applied, scored against that subset's own reference (no leakage): sum|Is-Iref|/sum|Iref|.
+    // Scoring on the studentized deviation instead would let the slope "improve" the held-out chi^2 by
+    // reshaping sigma - it scales sigma up on late / high-angle frames, down-weighting them - without
+    // actually tightening the intensities; a fractional metric cannot be gamed that way. This mirrors the
+    // absorption / modulation gate in ApplyCellSurface (which was switched off studentized chi^2 for the
+    // same reason); the reference is still the inverse-variance mean.
+    auto subset_disagreement = [&](int parity, double slope) -> double {
         std::vector<double> sw(n_groups, 0.0), swI(n_groups, 0.0);
         for (const auto &o : fulls) {
             if (!usable(o) || (o.frame & 1) != parity) continue;
@@ -604,17 +610,15 @@ void RotationScaleMerge::RefineDecay(int n_groups) {
             const double w = 1.0 / (sc * sc);
             sw[o.group] += w; swI[o.group] += w * Is;
         }
-        double sum = 0.0; size_t n = 0;
+        double num = 0.0, den = 0.0;
         for (const auto &o : fulls) {
             if (!usable(o) || (o.frame & 1) != parity || sw[o.group] <= 0.0) continue;
             const double Iref = swI[o.group] / sw[o.group];
             const double Is = static_cast<double>(o.I) * o.corr * decay_factor(o, slope);
-            const double sc = static_cast<double>(o.sigma) * o.corr * decay_factor(o, slope);
-            if (!std::isfinite(Iref) || !(sc > 0.0)) continue;
-            const double dd = (Is - Iref) / sc;
-            sum += dd * dd; ++n;
+            if (!std::isfinite(Iref) || Iref <= 0.0) continue;
+            num += std::abs(Is - Iref); den += Iref;
         }
-        return n > 0 ? sum / static_cast<double>(n) : 0.0;
+        return den > 0.0 ? num / den : 0.0;
     };
 
     // A meaningful radiation-damage relative-B accrues several A^2 across a dataset. Below a physical floor
@@ -631,8 +635,8 @@ void RotationScaleMerge::RefineDecay(int n_groups) {
     }
     // And require the slope to cross-validate by a clear margin: fit on even frames, score the held-out
     // odd equivalents (and vice versa). Real damage generalizes; a fluke does not.
-    const double base = subset_chi2(1, 0.0) + subset_chi2(0, 0.0);
-    const double gain = base - (subset_chi2(1, fit_slope(0)) + subset_chi2(0, fit_slope(1)));
+    const double base = subset_disagreement(1, 0.0) + subset_disagreement(0, 0.0);
+    const double gain = base - (subset_disagreement(1, fit_slope(0)) + subset_disagreement(0, fit_slope(1)));
     if (!(gain > CV_MIN_RELATIVE_GAIN * base)) {
         logger.Info("Decay correction: not cross-validated (dB = {:.2f} A^2, held-out gain {:.1f}%, skipped)",
                     total_delta_B, 100.0 * gain / std::max(base, 1e-30));