diff --git a/rugnux/Rugnux.h b/rugnux/Rugnux.h
index cf08a2cee..4af5023fd 100644
--- a/rugnux/Rugnux.h
+++ b/rugnux/Rugnux.h
@@ -839,10 +839,11 @@ bool ArmsHoldSameLattice(const ProcessResult &a, const ProcessResult &b);
 std::string MeasuredCentreWins(const ProcessResult &file_arm, const ProcessResult &measured_arm);
 // The search-merge numbers of a pass, as MeasuredCentreWins reads them.
 SearchMergeSignal SearchSignal(const ProcessResult &r);
-// Whether the measured centre loses on every line MeasuredCentreWins draws by more than `sigmas`
-// times that line's noise - the Poisson noise of a count, the standard error of a CC1/2 - with both
-// arms merged from the same frames. The run asks it at the near-tie distance of its first passes
-// (FirstPassRun::NEAR_TIE_SIGMA); anything else, a near tie or a win, is decided on all the frames.
+// Whether the measured centre is behind on the strong-reflection count by more than `sigmas` times
+// its Poisson noise - and, on the same lattice, also behind on the inner-shell CC1/2 line
+// MeasuredCentreWins draws - with both arms merged from the same frames. The run asks it at the
+// near-tie distance of its first passes (FirstPassRun::NEAR_TIE_SIGMA); anything else, a near tie or
+// a win, is decided on all the frames.
 bool MeasuredCentreClearlyLoses(const SearchMergeSignal &file_arm, const SearchMergeSignal &measured_arm,
                                 bool same_lattice, double sigmas);
 
diff --git a/rugnux/RugnuxPasses.cpp b/rugnux/RugnuxPasses.cpp
index a0776553b..7256ab640 100644
--- a/rugnux/RugnuxPasses.cpp
+++ b/rugnux/RugnuxPasses.cpp
@@ -89,6 +89,9 @@ namespace {
     // How much better the alternative beam centre's search merge has to be for the run to move off
     // the centre the file gave it (the beam-centre arbitration in RunAllPasses).
     constexpr double MAX_CC_HALF_LOSS = 0.05;
+    // How far, in Poisson sigmas, the alternative's strong-reflection count has to be ahead on the same
+    // lattice - the near-tie distance of the first passes (FirstPassRun::NEAR_TIE_SIGMA).
+    constexpr double STRONG_COUNT_SIGMAS = 2.0;
 
     // Say so rather than printing a zero that reads as a measurement. ("not compared", not "not
     // measured": completeness IS measured and reported; what did not happen is its use in arbitrating
@@ -292,7 +295,11 @@ std::string MeasuredCentreWins(const ProcessResult &file_arm, const ProcessResul
     // two arms here integrate with the same settings, so the geometry is the only difference, and the
     // file's centre has no standing the measured one lacks - neither indexed the whole spot list.
     if (ArmsHoldSameLattice(file_arm, measured_arm)) {
-        if (measured_arm.search_merge_strong_reflections > file_arm.search_merge_strong_reflections)
+        // Ahead by more than the Poisson noise of the two counts; within it the file's centre stays.
+        const int64_t strong_m = measured_arm.search_merge_strong_reflections;
+        const int64_t strong_f = file_arm.search_merge_strong_reflections;
+        if (static_cast<double>(strong_m - strong_f)
+            > STRONG_COUNT_SIGMAS * std::sqrt(static_cast<double>(strong_m + strong_f)))
             return fmt::format("the measured centre measures more of the crystal - {} reflections at "
                                "I/sigma >= 2 before corrections against {}",
                                measured_arm.search_merge_strong_reflections,
@@ -331,15 +338,19 @@ bool MeasuredCentreClearlyLoses(const SearchMergeSignal &file_arm, const SearchM
     const auto loses = [sigmas](double margin, double noise) {
         return std::isfinite(margin) && std::isfinite(noise) && margin < -sigmas * noise;
     };
-    if (same_lattice)
-        return loses(static_cast<double>(measured_arm.strong - file_arm.strong),
-                     std::sqrt(static_cast<double>(measured_arm.strong + file_arm.strong)))
-               && loses(measured_arm.inner_cc_half - file_arm.inner_cc_half - MAX_CC_HALF_LOSS,
-                        std::hypot(cc_se(measured_arm.inner_cc_half, measured_arm.inner_reflections),
-                                   cc_se(file_arm.inner_cc_half, file_arm.inner_reflections)));
-    return loses(measured_arm.cc_half - file_arm.cc_half - MAX_CC_HALF_LOSS,
-                 std::hypot(cc_se(measured_arm.cc_half, measured_arm.reflections),
-                            cc_se(file_arm.cc_half, file_arm.reflections)));
+    // Both arms integrate the same frames with the same settings, and the strong reflections are
+    // counted on the primitive lattice, so the counts compare across lattice classes as well. The
+    // subset's CC1/2 does not decide the different-lattice case: a third of the frames in blocks merges
+    // to a multiplicity near one in P1, its CC1/2 then reads the relative scale of the blocks, and
+    // measured it moved by 0.07 with how far the scaling loop ran - fifty times its Fisher noise.
+    const bool behind_on_signal = loses(static_cast<double>(measured_arm.strong - file_arm.strong),
+                                        std::sqrt(static_cast<double>(measured_arm.strong + file_arm.strong)));
+    if (!same_lattice)
+        return behind_on_signal;
+    return behind_on_signal
+           && loses(measured_arm.inner_cc_half - file_arm.inner_cc_half - MAX_CC_HALF_LOSS,
+                    std::hypot(cc_se(measured_arm.inner_cc_half, measured_arm.inner_reflections),
+                               cc_se(file_arm.inner_cc_half, file_arm.inner_reflections)));
 }
 
 ProcessResult Rugnux::RunAllPasses(RugnuxObserver *observer) {
diff --git a/tests/RugnuxTest.cpp b/tests/RugnuxTest.cpp
index 8308608fa..c85c7b86e 100644
--- a/tests/RugnuxTest.cpp
+++ b/tests/RugnuxTest.cpp
@@ -645,6 +645,8 @@ TEST_CASE("MeasuredCentreWins", "[process]") {
     // Same lattice, same CC1/2: the arm that measured more signal wins, whichever centre it is...
     CHECK(!MeasuredCentreWins(file, arm(swapped, tri, 0.87, 9500)).empty());
     CHECK(MeasuredCentreWins(file, arm(swapped, tri, 0.87, 8500)).empty());
+    // ...but not by less than twice the Poisson noise of the two counts (sqrt(18100) = 135)...
+    CHECK(MeasuredCentreWins(file, arm(swapped, tri, 0.87, 9100)).empty());
     // ...a tie keeps the file's centre, and a higher CC1/2 alone does not move it.
     CHECK(MeasuredCentreWins(file, arm(swapped, tri, 0.99, 9000)).empty());
 
@@ -676,12 +678,15 @@ TEST_CASE("MeasuredCentreClearlyLoses", "[process]") {
     m.inner_reflections = 20;
     CHECK_FALSE(MeasuredCentreClearlyLoses(inconsistent, m, true, 2.0));
 
-    // Different lattices: the CC1/2 line, 0.05 above the file's centre.
+    // Different lattices: the count alone - the subset's CC1/2 is not read.
     SearchMergeSignal d = file;
-    d.cc_half = 0.80;
+    d.strong = 3000;
+    d.cc_half = 0.99;
     CHECK(MeasuredCentreClearlyLoses(file, d, false, 2.0));
-    d.cc_half = 0.95;   // exactly on the line
+    d.strong = 8900;   // 100 behind: within two sigmas
+    d.cc_half = 0.50;
     CHECK_FALSE(MeasuredCentreClearlyLoses(file, d, false, 2.0));
-    // Nothing measured is no loss: it is decided on all the frames.
-    CHECK_FALSE(MeasuredCentreClearlyLoses(file, SearchMergeSignal{}, false, 2.0));
+    // A merge with no strong reflection in it measured none of the crystal. (An arm without a merge
+    // is never asked: RunAllPasses judges it on all the frames.)
+    CHECK(MeasuredCentreClearlyLoses(file, SearchMergeSignal{}, false, 2.0));
 }