diff --git a/image_analysis/scale_merge/ResolutionCutoff.cpp b/image_analysis/scale_merge/ResolutionCutoff.cpp
index 65df0f44..675f645e 100644
--- a/image_analysis/scale_merge/ResolutionCutoff.cpp
+++ b/image_analysis/scale_merge/ResolutionCutoff.cpp
@@ -140,9 +140,13 @@ ResolutionCutoffResult ComputeCCHalfLogisticCutoff(const std::vector<MergedRefle
         if (step_s0 < 1e-6 * s_range && step_k < 1e-6 * best_k) break;
     }
 
-    // s where the fitted CC1/2 crosses cc_target, then "one shell too far".
+    // s where the fitted CC1/2 crosses cc_target, then "one shell too far". The extension is one
+    // reported-shell width, measured over the range that is actually kept and reported (low-res
+    // plateau -> the fall-off crossing), NOT the full measured range: when the detector reaches far
+    // past where the crystal diffracts (a high-res-configured detector on a low-res crystal), the
+    // full range is dominated by high-res noise, so s_range/10 would be a huge over-extension.
     const double s_cross = s0 + std::log(1.0 / cc_target - 1.0) / k;
-    const double delta_s = s_range / SHELLS_FOR_EXTENSION;
+    const double delta_s = (s_cross - s_lo) / SHELLS_FOR_EXTENSION;
     const double s_final = s_cross + delta_s;
 
     // No cut if the fall-off is beyond the measured edge (CC1/2 still healthy at the highest s).