diff --git a/rugnux/Rugnux.cpp b/rugnux/Rugnux.cpp
index 805a7369b..6289bfda4 100644
--- a/rugnux/Rugnux.cpp
+++ b/rugnux/Rugnux.cpp
@@ -5023,8 +5023,11 @@ ProcessResult Rugnux::RunPipeline(RugnuxObserver *observer, bool write_output, b
                 // centring implies, and the pass then merges in P1 (measured on a C-centred monoclinic
                 // crystal). Only this direction - a centring pass 2 finds where pass 1 had none is the
                 // refined geometry doing its job, and the intensity test in the search confirms it.
+                // Same primitive metric only: a pass 2 that lands on a primitive cell several times
+                // smaller is a different lattice - pass 1's supercell corrected - not a dropped centring.
                 const bool lost_centring = prepass_result_->search_result.centering != 'P'
-                                        && best.result->search_result.centering == 'P';
+                                        && best.result->search_result.centering == 'P'
+                                        && v1 > 1.0 && std::abs(v2 / v1 - 1.0) < 0.1;
                 if (supercell || lost_centring) {
                     logger.Info("Two-pass: that lattice ({}-centred, volume {:.0f} A^3) disagrees with "
                                 "pass 1 ({}-centred, {:.0f} A^3) - integrating with pass-1's lattice instead",