diff --git a/rtengine/improcfun.cc b/rtengine/improcfun.cc
index 013a629fd..748e46187 100644
--- a/rtengine/improcfun.cc
+++ b/rtengine/improcfun.cc
@@ -5782,8 +5782,8 @@ void ImProcFunctions::lab2rgb (const LabImage &src, Imagefloat &dst, const Glib:
 */
 void ImProcFunctions::colorToningLabGrid(LabImage *lab, int xstart, int xend, int ystart, int yend, bool MultiThread)
 {
-    const float factor = ColorToningParams::LABGRID_CORR_MAX * 3.f;
-    const float scaling = ColorToningParams::LABGRID_CORR_SCALE;
+    const float factor = 3.f;
+    const float scaling = 3.f;
     float a_scale = (params->colorToning.labgridAHigh - params->colorToning.labgridALow) / factor / scaling;
     float a_base = params->colorToning.labgridALow / scaling;
     float b_scale = (params->colorToning.labgridBHigh - params->colorToning.labgridBLow) / factor / scaling;
diff --git a/rtengine/iplabregions.cc b/rtengine/iplabregions.cc
index 3854570d3..fe3325b1d 100644
--- a/rtengine/iplabregions.cc
+++ b/rtengine/iplabregions.cc
@@ -33,7 +33,7 @@ void ImProcFunctions::labColorCorrectionRegions(LabImage *lab)
         return;
     }
 
-    const float factor = ColorToningParams::LABGRID_CORR_MAX * 3.f;
+    const float factor = 3.f;
     const float scaling = 1.f;
 
     int n = params->colorToning.labregions.size();
diff --git a/rtgui/colortoning.cc b/rtgui/colortoning.cc
index c296a6b27..28b2a59ea 100644
--- a/rtgui/colortoning.cc
+++ b/rtgui/colortoning.cc
@@ -596,7 +596,7 @@ void ColorToning::read (const ProcParams* pp, const ParamsEdited* pedited)
 
     lastLumamode = pp->colorToning.lumamode;
 
-    labgrid->setParams(pp->colorToning.labgridALow, pp->colorToning.labgridBLow, pp->colorToning.labgridAHigh, pp->colorToning.labgridBHigh, false);
+    labgrid->setParams(pp->colorToning.labgridALow / ColorToningParams::LABGRID_CORR_MAX, pp->colorToning.labgridBLow / ColorToningParams::LABGRID_CORR_MAX, pp->colorToning.labgridAHigh / ColorToningParams::LABGRID_CORR_MAX, pp->colorToning.labgridBHigh / ColorToningParams::LABGRID_CORR_MAX, false);
 
     if (pedited && !pedited->colorToning.method) {
         method->set_active (5);
@@ -667,6 +667,10 @@ void ColorToning::write (ProcParams* pp, ParamsEdited* pedited)
     pp->colorToning.strength               = strength->getIntValue();
 
     labgrid->getParams(pp->colorToning.labgridALow, pp->colorToning.labgridBLow, pp->colorToning.labgridAHigh, pp->colorToning.labgridBHigh);
+    pp->colorToning.labgridALow *= ColorToningParams::LABGRID_CORR_MAX;
+    pp->colorToning.labgridAHigh *= ColorToningParams::LABGRID_CORR_MAX;
+    pp->colorToning.labgridBLow *= ColorToningParams::LABGRID_CORR_MAX;
+    pp->colorToning.labgridBHigh *= ColorToningParams::LABGRID_CORR_MAX;
 
     labRegionGet(labRegionSelected);
     labRegionShow(labRegionSelected, true);
@@ -778,7 +782,7 @@ void ColorToning::setDefaults (const ProcParams* defParams, const ParamsEdited*
     hlColSat->setDefault<int> (defParams->colorToning.hlColSat);
     shadowsColSat->setDefault<int> (defParams->colorToning.shadowsColSat);
     strength->setDefault (defParams->colorToning.strength);
-    labgrid->setDefault(defParams->colorToning.labgridALow, defParams->colorToning.labgridBLow, defParams->colorToning.labgridAHigh, defParams->colorToning.labgridBHigh);
+    labgrid->setDefault(defParams->colorToning.labgridALow / ColorToningParams::LABGRID_CORR_MAX, defParams->colorToning.labgridBLow / ColorToningParams::LABGRID_CORR_MAX, defParams->colorToning.labgridAHigh / ColorToningParams::LABGRID_CORR_MAX, defParams->colorToning.labgridBHigh / ColorToningParams::LABGRID_CORR_MAX);
     
 
     if (pedited) {
diff --git a/rtgui/labgrid.cc b/rtgui/labgrid.cc
index fde03194b..fe984ffe9 100644
--- a/rtgui/labgrid.cc
+++ b/rtgui/labgrid.cc
@@ -44,7 +44,12 @@ using rtengine::Color;
 bool LabGrid::notifyListener()
 {
     if (listener) {
-        listener->panelChanged(evt, Glib::ustring::compose(evtMsg, int(high_a), int(high_b), int(low_a), int(low_b)));
+        const auto round =
+            [](float v) -> float
+            {
+                return int(v * 1000) / 1000.f;
+            };
+        listener->panelChanged(evt, Glib::ustring::compose(evtMsg, round(high_a), round(high_b), round(low_a), round(low_b)));
     }
     return false;
 }
@@ -76,8 +81,8 @@ void LabGrid::getParams(double &la, double &lb, double &ha, double &hb) const
 
 void LabGrid::setParams(double la, double lb, double ha, double hb, bool notify)
 {
-    const double lo = -rtengine::ColorToningParams::LABGRID_CORR_MAX;
-    const double hi = rtengine::ColorToningParams::LABGRID_CORR_MAX;
+    const double lo = -1.0;
+    const double hi = 1.0;
     low_a = rtengine::LIM(la, lo, hi);
     low_b = rtengine::LIM(lb, lo, hi);
     high_a = rtengine::LIM(ha, lo, hi);
@@ -172,7 +177,7 @@ bool LabGrid::on_draw(const ::Cairo::RefPtr<Cairo::Context> &crf)
         cr->translate(0, height);
         cr->scale(1., -1.);
         const int cells = 8;
-        float step = rtengine::ColorToningParams::LABGRID_CORR_MAX / float(cells/2);
+        float step = 1.f / float(cells/2);
         for (int j = 0; j < cells; j++) {
             for (int i = 0; i < cells; i++) {
                 float R, G, B;
@@ -192,10 +197,10 @@ bool LabGrid::on_draw(const ::Cairo::RefPtr<Cairo::Context> &crf)
         // drawing the connection line
         cr->set_antialias(Cairo::ANTIALIAS_DEFAULT);
         float loa, hia, lob, hib;
-        loa = .5f * (width + width * low_a / rtengine::ColorToningParams::LABGRID_CORR_MAX);
-        hia = .5f * (width + width * high_a / rtengine::ColorToningParams::LABGRID_CORR_MAX);
-        lob = .5f * (height + height * low_b / rtengine::ColorToningParams::LABGRID_CORR_MAX);
-        hib = .5f * (height + height * high_b / rtengine::ColorToningParams::LABGRID_CORR_MAX);
+        loa = .5f * (width + width * low_a);
+        hia = .5f * (width + width * high_a);
+        lob = .5f * (height + height * low_b);
+        hib = .5f * (height + height * high_b);
         cr->set_line_width(2.);
         cr->set_source_rgb(0.6, 0.6, 0.6);
         cr->move_to(loa, lob);
@@ -279,11 +284,11 @@ bool LabGrid::on_motion_notify_event(GdkEventMotion *event)
     const float mb = (2.0 * mouse_y - height) / (float)height;
     if (isDragged) {
         if (litPoint == LOW) {
-            low_a = ma * rtengine::ColorToningParams::LABGRID_CORR_MAX;
-            low_b = mb * rtengine::ColorToningParams::LABGRID_CORR_MAX;
+            low_a = ma;
+            low_b = mb;
         } else if (litPoint == HIGH) {
-            high_a = ma * rtengine::ColorToningParams::LABGRID_CORR_MAX;
-            high_b = mb * rtengine::ColorToningParams::LABGRID_CORR_MAX;
+            high_a = ma;
+            high_b = mb;
         }
         edited = true;
         grab_focus();
@@ -295,10 +300,10 @@ bool LabGrid::on_motion_notify_event(GdkEventMotion *event)
         queue_draw();
     } else {
         litPoint = NONE;
-        float la = low_a / rtengine::ColorToningParams::LABGRID_CORR_MAX;
-        float lb = low_b / rtengine::ColorToningParams::LABGRID_CORR_MAX;
-        float ha = high_a / rtengine::ColorToningParams::LABGRID_CORR_MAX;
-        float hb = high_b / rtengine::ColorToningParams::LABGRID_CORR_MAX;
+        float la = low_a;
+        float lb = low_b;
+        float ha = high_a;
+        float hb = high_b;
         const float thrs = 0.05f;
         const float distlo = (la - ma) * (la - ma) + (lb - mb) * (lb - mb);
         const float disthi = (ha - ma) * (ha - ma) + (hb - mb) * (hb - mb);