Add option to fix E/D ratio

Work around for slicers producing buggy gcode.

Marlin/example_configurations/K8400/Configuration_adv.h

@@ -589,6 +589,31 @@
 
 #if ENABLED(LIN_ADVANCE)
   #define LIN_ADVANCE_K 75
+
+  /**
+   * Some Slicers produce Gcode with randomly jumping extrusion widths occasionally.
+   * For example within a 0.4mm perimeter it may produce a single segment of 0.05mm width.
+   * While this is harmless for normal printing (the fluid nature of the filament will
+   * close this very, very tiny gap), it throws off the LIN_ADVANCE pressure adaption.
+   *
+   * For this case LIN_ADVANCE_E_D_RATIO can be used to set the extrusion:distance ratio
+   * to a fixed value. Note that using a fixed ratio will lead to wrong nozzle pressures
+   * if the slicer is using variable widths or layer heights within one print!
+   *
+   * This option sets the default E:D ratio at startup. Use `M905` to override this value.
+   *
+   * Example: `M905 W0.4 H0.2 D1.75`, where:
+   *   - W is the extrusion width in mm
+   *   - H is the layer height in mm
+   *   - D is the filament diameter in mm
+   *
+   * Set to 0 to auto-detect the ratio based on given Gcode G1 print moves.
+   *
+   * Slic3r (including Prusa Slic3r) produces Gcode compatible with the automatic mode.
+   * Cura (as of this writing) may produce Gcode incompatible with the automatic mode.
+   */
+  #define LIN_ADVANCE_E_D_RATIO 0 // The calculated ratio (or 0) according to the formula W * H / ((D / 2) ^ 2 * PI)
+                                  // Example: 0.4 * 0.2 / ((1.75 / 2) ^ 2 * PI) = 0.033260135
 #endif
 
 // @section leveling