Adjustable Z probe offset, via custom M-code

Marlin/Configuration.h

@@ -459,6 +459,15 @@ const bool Z_MAX_ENDSTOP_INVERTING = true; // set to true to invert the logic of
 //=============================Additional Features===========================
 //===========================================================================
 
+// Custom M code points
+#define CUSTOM_M_CODES
+#ifdef CUSTOM_M_CODES
+  #define CUSTOM_M_CODE_SET_Z_PROBE_OFFSET 851
+  #define Z_PROBE_OFFSET_RANGE_MIN -2
+  #define Z_PROBE_OFFSET_RANGE_MAX 0
+#endif
+
+
 // EEPROM
 // The microcontroller can store settings in the EEPROM, e.g. max velocity...
 // M500 - stores parameters in EEPROM

Marlin/Marlin_main.cpp

@@ -2719,6 +2719,42 @@ void process_commands()
     }
     break;
     #endif
+
+    #ifdef CUSTOM_M_CODE_SET_Z_PROBE_OFFSET
+    case CUSTOM_M_CODE_SET_Z_PROBE_OFFSET:
+    {
+      float value;
+      if (code_seen('Z'))
+      {
+        value = code_value();
+        if ((Z_PROBE_OFFSET_RANGE_MIN <= value) && (value <= Z_PROBE_OFFSET_RANGE_MAX))
+        {
+          zprobe_zoffset = -value; // compare w/ line 278 of ConfigurationStore.cpp
+          SERIAL_ECHO_START;
+          SERIAL_ECHOLNPGM("Z probe offset has been set");
+          SERIAL_PROTOCOLLN("");
+        }
+        else
+        {
+          SERIAL_ECHO_START;
+          SERIAL_ECHOPGM("Invalid z-probe value.  Must be between ");
+          SERIAL_ECHO(Z_PROBE_OFFSET_RANGE_MIN);
+          SERIAL_ECHOPGM(" and ");
+          SERIAL_ECHO(Z_PROBE_OFFSET_RANGE_MAX);
+          SERIAL_PROTOCOLLN("");
+        }
+      }
+      else
+      {
+          SERIAL_ECHO_START;
+          SERIAL_ECHOLNPGM("Z probe offset is currently ");
+          SERIAL_ECHO(-zprobe_zoffset);
+          SERIAL_PROTOCOLLN("");
+      }
+      break;
+    }
+    #endif // CUSTOM_M_CODE_SET_Z_PROBE_OFFSET
+
     #ifdef FILAMENTCHANGEENABLE
     case 600: //Pause for filament change X[pos] Y[pos] Z[relative lift] E[initial retract] L[later retract distance for removal]
     {