Reduce E_D_ratio code

Marlin/Marlin_main.cpp

    */
   inline void gcode_M905() {
     stepper.synchronize();
-    
-    float newD = -1;
-    float newW = -1;
-    float newH = -1;
-    
-    if (code_seen('K')) {
-      float newK = code_value_float();
-      if (newK >= 0.0)
-        planner.set_extruder_advance_k(newK);
-    }
+
+    const float newK = code_seen('K') ? code_value_float() : -1,
+                newD = code_seen('D') ? code_value_float() : -1,
+                newW = code_seen('W') ? code_value_float() : -1,
+                newH = code_seen('H') ? code_value_float() : -1;
+
+    if (newK >= 0.0) planner.set_extruder_advance_k(newK);
 
     SERIAL_ECHO_START;
-    SERIAL_ECHOPAIR("Advance factor: ", planner.get_extruder_advance_k());
-    SERIAL_EOL;
-    
-    if (code_seen('D'))
-      newD = code_value_float();
-    if (code_seen('W'))
-      newW = code_value_float();
-    if (code_seen('H'))
-      newH = code_value_float();
-
-    if (newD > 0 && newW > 0 && newH > 0) {
-      float E_D_ratio = newW * newH / (sq(newD / 2) * M_PI);
-      planner.set_E_D_ratio(E_D_ratio);
-      SERIAL_ECHO_START;
-      SERIAL_ECHOPAIR("E/D ratio: ", E_D_ratio);
-      SERIAL_EOL;
-    }
-    else if (newD != -1 || newW != -1 || newH != -1) {
-      planner.set_E_D_ratio(0);
+    SERIAL_ECHOLNPAIR("Advance factor: ", planner.get_extruder_advance_k());
+
+    if (newD >= 0 || newW >= 0 || newH >= 0) {
+      const float ratio = (!newD || !newW || !newH) ? 0 : (newW * newH) / (sq(newD * 0.5) * M_PI);
+      planner.set_advance_ed_ratio(ratio);
       SERIAL_ECHO_START;
-      SERIAL_ECHOPGM("E/D ratio: Automatic");
-      SERIAL_EOL;
+      SERIAL_ECHOPGM("E/D ratio: ");
+      if (ratio) SERIAL_ECHOLN(ratio); else SERIAL_ECHOLNPGM("Automatic");
     }
   }
 #endif

Marlin/planner.cpp

 
 #if ENABLED(LIN_ADVANCE)
   float Planner::extruder_advance_k = LIN_ADVANCE_K,
-        Planner::E_D_ratio = LIN_ADVANCE_E_D_RATIO,
+        Planner::advance_ed_ratio = LIN_ADVANCE_E_D_RATIO,
         Planner::position_float[NUM_AXIS] = { 0 };
 #endif
 
                             && extruder_advance_k
                             && (uint32_t)esteps != block->step_event_count
                             && de_float > 0.0;
-    if (block->use_advance_lead) {
-      // Check if we should use the fixed E_D_ratio
-      if (UNEAR_ZERO(E_D_ratio)) {
-        block->abs_adv_steps_multiplier8 = lround(extruder_advance_k * (de_float / mm_D_float) * block->nominal_speed / (float)block->nominal_rate * axis_steps_per_mm[E_AXIS_N] * 256.0);
-      }
-      else {
-        block->abs_adv_steps_multiplier8 = lround(extruder_advance_k * E_D_ratio * block->nominal_speed / (float)block->nominal_rate * axis_steps_per_mm[E_AXIS_N] * 256.0);
-      }
-    }
+    if (block->use_advance_lead)
+      block->abs_adv_steps_multiplier8 = lround(
+        extruder_advance_k
+        * (UNEAR_ZERO(advance_ed_ratio) ? de_float / mm_D_float : advance_ed_ratio) // Use the fixed ratio, if set
+        * (block->nominal_speed / (float)block->nominal_rate)
+        * axis_steps_per_mm[E_AXIS_N] * 256.0
+      );
 
   #elif ENABLED(ADVANCE)
 

Marlin/planner.h

     #if ENABLED(LIN_ADVANCE)
       static float position_float[NUM_AXIS];
       static float extruder_advance_k;
-      static float E_D_ratio;
+      static float advance_ed_ratio;
     #endif
 
     #if ENABLED(ULTRA_LCD)
     #if ENABLED(LIN_ADVANCE)
       static void set_extruder_advance_k(const float &k) { extruder_advance_k = k; };
       static float get_extruder_advance_k() { return extruder_advance_k; };
-      static void set_E_D_ratio(const float &ratio) { E_D_ratio = ratio; };
+      static void set_advance_ed_ratio(const float &ratio) { advance_ed_ratio = ratio; };
     #endif
 
     /**