diff --git a/image_analysis/IndexAndRefine.cpp b/image_analysis/IndexAndRefine.cpp
index 76f972d8..6dfbb683 100644
--- a/image_analysis/IndexAndRefine.cpp
+++ b/image_analysis/IndexAndRefine.cpp
@@ -276,7 +276,7 @@ std::optional<RotationIndexerResult> IndexAndRefine::Finalize() {
     return {};
 }
 
-std::optional<ScaleResult> IndexAndRefine::ScaleAllImages(const ScaleMergeOptions &opts) {
+std::optional<ScalingResult> IndexAndRefine::ScaleAllImages(const ScaleMergeOptions &opts) {
     size_t nrefl = 0;
     for (const auto &i: reflections)
         nrefl += i.size();
diff --git a/image_analysis/IndexAndRefine.h b/image_analysis/IndexAndRefine.h
index 2a5e5b86..4ef24b13 100644
--- a/image_analysis/IndexAndRefine.h
+++ b/image_analysis/IndexAndRefine.h
@@ -62,7 +62,7 @@ public:
     void ProcessImage(DataMessage &msg, const SpotFindingSettings &settings, const CompressedImage &image, BraggPrediction &prediction);
 
 
-    std::optional<ScaleResult> ScaleAllImages(const ScaleMergeOptions &opts = {});
+    std::optional<ScalingResult> ScaleAllImages(const ScaleMergeOptions &opts = {});
 
     /// Run scale-and-merge on accumulated reflections to refine per-image
     /// mosaicity (and optionally B-factors / scale factors).
diff --git a/image_analysis/scale_merge/CMakeLists.txt b/image_analysis/scale_merge/CMakeLists.txt
index 3f06312e..2bde4a67 100644
--- a/image_analysis/scale_merge/CMakeLists.txt
+++ b/image_analysis/scale_merge/CMakeLists.txt
@@ -6,5 +6,6 @@ ADD_LIBRARY(JFJochScaleMerge ScaleAll.cpp ScaleAll.h FrenchWilson.cpp FrenchWils
         ScaleOnTheFly.cpp
         ScaleOnTheFly.h
         HKLKey.cpp
-        HKLKey.h)
+        HKLKey.h
+        ScalingResult.h)
 TARGET_LINK_LIBRARIES(JFJochScaleMerge Ceres::ceres Eigen3::Eigen JFJochCommon)
\ No newline at end of file
diff --git a/image_analysis/scale_merge/ScaleAll.cpp b/image_analysis/scale_merge/ScaleAll.cpp
index f83514dd..310392c1 100644
--- a/image_analysis/scale_merge/ScaleAll.cpp
+++ b/image_analysis/scale_merge/ScaleAll.cpp
@@ -410,7 +410,7 @@ namespace {
     }
 }
 
-ScaleResult ScaleAll(std::vector<std::vector<Reflection> > &observations,
+ScalingResult ScaleAll(std::vector<std::vector<Reflection> > &observations,
                      const ScaleMergeOptions &opt) {
     if (opt.image_cluster <= 0)
         throw std::invalid_argument("image_cluster must be positive");
@@ -442,7 +442,7 @@ ScaleResult ScaleAll(std::vector<std::vector<Reflection> > &observations,
 
     CalcCorrections(observations, opt, G, mosaicity, R_sq);
 
-    ScaleResult out{};
+    ScalingResult out{};
 
     out.image_scale_g.resize(observations.size(), NAN);
     out.mosaicity_deg.resize(observations.size(), NAN);
diff --git a/image_analysis/scale_merge/ScaleAll.h b/image_analysis/scale_merge/ScaleAll.h
index f06aaa26..45766914 100644
--- a/image_analysis/scale_merge/ScaleAll.h
+++ b/image_analysis/scale_merge/ScaleAll.h
@@ -4,13 +4,7 @@
 #pragma once
 
 #include "Merge.h"
+#include "ScalingResult.h"
 
 
-struct ScaleResult {
-    std::vector<float> image_scale_g;
-    std::vector<float> mosaicity_deg;
-    std::vector<float> image_bfactor_Ang2;
-};
-
-ScaleResult ScaleAll(std::vector<std::vector<Reflection>>& observations,
-                                               const ScaleMergeOptions& opt = {});
\ No newline at end of file
+ScalingResult ScaleAll(std::vector<std::vector<Reflection>>& observations, const ScaleMergeOptions& opt = {});
\ No newline at end of file
diff --git a/image_analysis/scale_merge/ScaleOnTheFly.cpp b/image_analysis/scale_merge/ScaleOnTheFly.cpp
index 5bf75560..f225d6cc 100644
--- a/image_analysis/scale_merge/ScaleOnTheFly.cpp
+++ b/image_analysis/scale_merge/ScaleOnTheFly.cpp
@@ -3,6 +3,52 @@
 
 #include "ScaleOnTheFly.h"
 
+#include <ceres/ceres.h>
+
+double SafeInv(double x, double fallback) {
+    if (!std::isfinite(x) || x == 0.0)
+        return fallback;
+    return 1.0 / x;
+}
+
+struct IntensityRotationResidual {
+    IntensityRotationResidual(const Reflection &r, double Itrue, double sigma)
+        : Iobs(r.I),
+          Itrue(Itrue),
+          weight(SafeInv(sigma, 1.0)),
+          delta_phi_deg(r.delta_phi_deg),
+          lp(SafeInv(r.rlp, 1.0)),
+          c1(r.zeta / std::sqrt(2.0)),
+          b_resolution_coeff(SafeInv(-r.d * r.d / 4.0, 0.0)) {
+    }
+
+    template<typename T>
+    bool operator()(const T *const G,
+                    const T *const B,
+                    const T *const mosaicity,
+                    const T *const wedge,
+                    T *residual) const {
+        const T partiality = T(1.0);
+        if (mosaicity > 0) {
+            const T half_wedge = wedge[0] / T(2.0);
+            const T arg_plus = T(delta_phi_deg + half_wedge) * T(c1) / mosaicity[0];
+            const T arg_minus = T(delta_phi_deg - half_wedge) * T(c1) / mosaicity[0];
+            partiality = (ceres::erf(arg_plus) - ceres::erf(arg_minus)) / T(2.0);
+        }
+        const T B_term = ceres::exp(T(B[0]) * b_resolution_coeff);
+        residual[0] = (G[0] * partiality * T(lp) * Itrue - T(Iobs)) * T(weight);
+        return true;
+    }
+
+    double Iobs;
+    double Itrue;
+    double weight;
+    double delta_phi_deg;
+    double lp;
+    double c1;
+    double b_resolution_coeff;
+};
+
 ScaleOnTheFly::ScaleOnTheFly(const std::vector<MergedReflection> &ref,
     std::optional<gemmi::SpaceGroup> sg,
     bool merge_friedel) : sg(sg), merge_friedel(merge_friedel) {
@@ -11,3 +57,7 @@ ScaleOnTheFly::ScaleOnTheFly(const std::vector<MergedReflection> &ref,
         reference_data[key] = r.I;
     }
 }
+
+std::optional<ScalingResult> ScaleOnTheFly::Scale(std::vector<Reflection> &reflections) {
+    
+}
diff --git a/image_analysis/scale_merge/ScaleOnTheFly.h b/image_analysis/scale_merge/ScaleOnTheFly.h
index 21b4a2ba..798698e0 100644
--- a/image_analysis/scale_merge/ScaleOnTheFly.h
+++ b/image_analysis/scale_merge/ScaleOnTheFly.h
@@ -5,6 +5,7 @@
 
 #include "HKLKey.h"
 #include "Merge.h"
+#include "ScalingResult.h"
 
 #include <map>
 
@@ -14,6 +15,6 @@ class ScaleOnTheFly {
     std::map<HKLKey, double> reference_data;
 public:
     ScaleOnTheFly(const std::vector<MergedReflection> &ref, std::optional<gemmi::SpaceGroup> sg, bool merge_friedel = true);
-    void Scale(std::vector<Reflection> &reflections);
+    std::optional<ScalingResult> Scale(std::vector<Reflection> &reflections);
 };
 
diff --git a/image_analysis/scale_merge/ScalingResult.h b/image_analysis/scale_merge/ScalingResult.h
new file mode 100644
index 00000000..e4701814
--- /dev/null
+++ b/image_analysis/scale_merge/ScalingResult.h
@@ -0,0 +1,12 @@
+// SPDX-FileCopyrightText: 2025 Filip Leonarski, Paul Scherrer Institute <filip.leonarski@psi.ch>
+// SPDX-License-Identifier: GPL-3.0-only
+
+#pragma once
+
+#include <vector>
+
+struct ScalingResult {
+    std::vector<float> image_scale_g;
+    std::vector<float> mosaicity_deg;
+    std::vector<float> image_bfactor_Ang2;
+};
\ No newline at end of file