diff --git a/image_analysis/geom_refinement/CellEsd.cpp b/image_analysis/geom_refinement/CellEsd.cpp
index 23340f18b..1c7924d9b 100644
--- a/image_analysis/geom_refinement/CellEsd.cpp
+++ b/image_analysis/geom_refinement/CellEsd.cpp
@@ -6,6 +6,7 @@
 #include <algorithm>
 #include <cmath>
 
+#include "../../common/JFJochMath.h"
 #include "../../common/ParallelFor.h"
 
 namespace {
@@ -37,7 +38,7 @@ namespace {
         return v;
     }
 
-    double ToDeg(double rad) { return rad * 180.0 / M_PI; }
+    double ToDeg(double rad) { return rad * 180.0 / PI; }
 }
 
 std::vector<Eigen::Matrix3d> MetricBasis(const gemmi::SpaceGroup &sg) {
@@ -206,6 +207,7 @@ std::optional<CellEsd> EstimateCellEsd(std::span<const CellObservation> obs, con
     out.theta_max_deg = ToDeg(std::asin(std::min(1.0, wavelength_A * q_max / 2.0)));
 
     // The same fit on blocks of consecutive frames: the scatter of their cells, over sqrt(blocks).
+    // Least-squares cell e.s.d.s alone are too small: following Herbstein (2000) Acta Cryst. B56, 547-557
     std::vector<std::array<double, 6>> block_cells;
     const int64_t span = static_cast<int64_t>(frame_max) - frame_min + 1;
     std::vector<std::vector<CellObservation>> block_obs(nblocks);