diff --git a/image_analysis/geom_refinement/XtalOptimizer.cpp b/image_analysis/geom_refinement/XtalOptimizer.cpp
index 7bd37af4..b7cdb04d 100644
--- a/image_analysis/geom_refinement/XtalOptimizer.cpp
+++ b/image_analysis/geom_refinement/XtalOptimizer.cpp
@@ -355,17 +355,18 @@ void LatticeToRodriguesAndLengths_Hex(const CrystalLattice &latt, double rod[3],
 // Extract rotation (Rodrigues), lengths (a,b,c) and beta (rad) for monoclinic (unique axis b).
 // Frame choice: e2 aligned with b; e1 from a projected orthogonal to e2; e3 = e1 x e2.
 void LatticeToRodriguesLengthsBeta_Mono(const CrystalLattice &latt,
-                                               double rod[3],
-                                               double lengths[3],
-                                               double &beta_rad) {
+                                        double rod[3],
+                                        double lengths[3],
+                                        double &beta_rad) {
     const Coord a = latt.Vec0();
     const Coord b = latt.Vec1();
     const Coord c = latt.Vec2();
 
-    Eigen::Vector3d A(a[0], a[1], a[2]);
-    Eigen::Vector3d Bv(b[0], b[1], b[2]);
-    Eigen::Vector3d C(c[0], c[1], c[2]);
+    const Eigen::Vector3d A(a[0], a[1], a[2]);
+    const Eigen::Vector3d Bv(b[0], b[1], b[2]);
+    const Eigen::Vector3d C(c[0], c[1], c[2]);
 
+    // Unit cell lengths
     const double a_len = A.norm();
     const double b_len = Bv.norm();
     const double c_len = C.norm();
@@ -374,41 +375,63 @@ void LatticeToRodriguesLengthsBeta_Mono(const CrystalLattice &latt,
     lengths[1] = b_len;
     lengths[2] = c_len;
 
-    // beta = angle between a and c
+    // Monoclinic beta = angle(a, c)
     double cos_beta = 0.0;
-    if (a_len > 0.0 && c_len > 0.0)
-        cos_beta = std::max(-1.0, std::min(1.0, A.dot(C) / (a_len * c_len)));
+    if (a_len > 1e-15 && c_len > 1e-15) {
+        cos_beta = A.dot(C) / (a_len * c_len);
+        cos_beta = std::clamp(cos_beta, -1.0, 1.0);
+    }
+
     beta_rad = std::acos(cos_beta);
 
-    // Recover R from the same forward model used in refinement:
-    // L ≈ R * B(beta) * D(a,b,c)  =>  R ≈ L * (B*D)^-1
+    // Protect against singular construction
+    const double sin_beta = std::max(std::abs(std::sin(beta_rad)), 1e-12);
+
+    // Canonical monoclinic basis:
+    //
+    // B =
+    // [ 1   0   cos(beta) ]
+    // [ 0   1        0     ]
+    // [ 0   0   sin(beta) ]
+    //
+    Eigen::Matrix3d Bmono = Eigen::Matrix3d::Zero();
+    Bmono(0,0) = 1.0;
+    Bmono(1,1) = 1.0;
+    Bmono(0,2) = std::cos(beta_rad);
+    Bmono(2,2) = sin_beta;
+
+    // Scale by lengths
+    Eigen::DiagonalMatrix<double,3> D(a_len, b_len, c_len);
+
+    // Ideal body-frame lattice
+    const Eigen::Matrix3d M = Bmono * D;
+
+    // Observed lattice
     Eigen::Matrix3d L;
     L.col(0) = A;
     L.col(1) = Bv;
     L.col(2) = C;
 
-    Eigen::Matrix3d Bmono = Eigen::Matrix3d::Identity();
-    Bmono(0, 2) = std::cos(beta_rad);
-    Bmono(2, 2) = std::sin(beta_rad);
-
-    Eigen::DiagonalMatrix<double, 3> D(lengths[0], lengths[1], lengths[2]);
-    Eigen::Matrix3d M = Bmono * D;
-
-    Eigen::Matrix3d R_est = Eigen::Matrix3d::Identity();
-    if (std::abs(M.determinant()) > 1e-15) {
-        R_est = L * M.inverse();
-    }
+    // Estimate rotation:
+    // R ≈ L * M^{-1}
+    Eigen::Matrix3d R_est = L * M.inverse();
 
+    // Project to nearest proper rotation matrix
     Eigen::JacobiSVD<Eigen::Matrix3d> svd(R_est, Eigen::ComputeFullU | Eigen::ComputeFullV);
+
     Eigen::Matrix3d R = svd.matrixU() * svd.matrixV().transpose();
+
+    // Enforce det(R)=+1
     if (R.determinant() < 0.0) {
         Eigen::Matrix3d U = svd.matrixU();
         U.col(2) *= -1.0;
         R = U * svd.matrixV().transpose();
     }
 
+    // Rodrigues vector
     Eigen::AngleAxisd aa(R);
-    Eigen::Vector3d r = aa.angle() * aa.axis();
+    const Eigen::Vector3d r = aa.angle() * aa.axis();
+
     rod[0] = r.x();
     rod[1] = r.y();
     rod[2] = r.z();