From 5027ce5973f094ebc18ec472b9b72dd94d630731 Mon Sep 17 00:00:00 2001
From: Scott Lahteine <thinkyhead@users.noreply.github.com>
Date: Fri, 11 Mar 2022 15:45:40 -0600
Subject: [PATCH] M423 E to set spacing as (e - s) / n

---
 Marlin/src/feature/x_twist.cpp       |  4 ++--
 Marlin/src/gcode/probe/M423.cpp      | 18 ++++++++++++++----
 Marlin/src/lcd/menu/menu_x_twist.cpp |  2 +-
 3 files changed, 17 insertions(+), 7 deletions(-)

diff --git a/Marlin/src/feature/x_twist.cpp b/Marlin/src/feature/x_twist.cpp
index 891009123986e..ce637de5aeeef 100644
--- a/Marlin/src/feature/x_twist.cpp
+++ b/Marlin/src/feature/x_twist.cpp
@@ -36,8 +36,8 @@ void XATC::reset() {
   constexpr float xzo[] = XATC_Z_OFFSETS;
   static_assert(COUNT(xzo) == XATC_MAX_POINTS, "XATC_Z_OFFSETS is the wrong size.");
   COPY(z_offset, xzo);
-  xatc.spacing = (probe.max_x() - probe.min_x()) / (XATC_MAX_POINTS - 1);
-  xatc.start = probe.min_x();
+  start = probe.min_x();
+  spacing = (probe.max_x() - start) / (XATC_MAX_POINTS - 1);
   enabled = true;
 }
 
diff --git a/Marlin/src/gcode/probe/M423.cpp b/Marlin/src/gcode/probe/M423.cpp
index 61b5db33069df..4d02feb119dc5 100644
--- a/Marlin/src/gcode/probe/M423.cpp
+++ b/Marlin/src/gcode/probe/M423.cpp
@@ -30,20 +30,23 @@
 
 #include "../gcode.h"
 #include "../../feature/x_twist.h"
+#include "../../module/probe.h"
 
 /**
  * M423: Set a Z offset for X-Twist (added to the mesh on future G29).
  *  M423 [R] [A<startx>] [I<interval>] [X<index> Z<offset>]
  *
  *    R         - Reset the twist compensation data
- *    A<linear> - Override the X twist starting X position
- *    I<linear> - Override the X twist X-spacing
+ *    A<linear> - Set the X twist starting X position
+ *    E<linear> - Set the X twist ending X position
+ *    I<linear> - Set the X twist X-spacing directly
  *    X<index>  - Index of a Z value in the list
  *    Z<linear> - A Z value to set
  */
 void GcodeSuite::M423() {
 
   bool do_report = true;
+  float new_spacing = 0;
 
   if (parser.seen_test('R')) {
     do_report = false;
@@ -52,11 +55,18 @@ void GcodeSuite::M423() {
   if (parser.seenval('A')) {
     do_report = false;
     xatc.start = parser.value_float();
+    new_spacing = (probe.max_x() - xatc.start) / (XATC_MAX_POINTS - 1);
   }
-  if (parser.seenval('I')) {
+  if (parser.seenval('E')) {
     do_report = false;
-    xatc.spacing = parser.value_float();
+    new_spacing = (parser.value_float() - xatc.start) / (XATC_MAX_POINTS - 1);
   }
+  else if (parser.seenval('I')) {
+    do_report = false;
+    new_spacing = parser.value_float();
+  }
+
+  if (new_spacing) xatx.spacing = new_spacing;
 
   if (parser.seenval('X')) {
     do_report = false;
diff --git a/Marlin/src/lcd/menu/menu_x_twist.cpp b/Marlin/src/lcd/menu/menu_x_twist.cpp
index 93f8de07ad465..ce46053dfc346 100644
--- a/Marlin/src/lcd/menu/menu_x_twist.cpp
+++ b/Marlin/src/lcd/menu/menu_x_twist.cpp
@@ -153,7 +153,7 @@ void xatc_wizard_goto_next_point() {
       measured_z = probe.probe_at_point(x, XATC_Y_POSITION, PROBE_PT_STOW);
       xatc.set_enabled(true);
       current_position += probe.offset_xy;
-      current_position.z = XATC_START_Z - probe.offset.z + measured_z;
+      current_position.z = (XATC_START_Z) - probe.offset.z + measured_z;
       line_to_current_position(MMM_TO_MMS(XY_PROBE_FEEDRATE));
       ui.wait_for_move = false;
     }