Update U20 config to help UBL toggle (#16471)

config/examples/Alfawise/U20-bltouch/Configuration.h

 /**
  * Normally G28 leaves leveling disabled on completion. Enable
  * this option to have G28 restore the prior leveling state.
+ * If false, use M420 S1 after G28 in your slicer print start gcode
  */
-//#define RESTORE_LEVELING_AFTER_G28
+#define RESTORE_LEVELING_AFTER_G28 false
 
 /**
  * Enable detailed logging of G28, G29, M48, etc.
 
   //#define MESH_EDIT_GFX_OVERLAY   // Display a graphics overlay while editing the mesh
 
-  #define MESH_INSET 1              // Set Mesh bounds as an inset region of the bed
-  #define GRID_MAX_POINTS_X 10      // Don't use more than 15 points per axis, implementation limited.
+  #define MESH_INSET 5              // Set Mesh bounds as an inset region of the bed
+  #define GRID_MAX_POINTS_X 5       // Don't use more than 15 points per axis, implementation limited.
   #define GRID_MAX_POINTS_Y GRID_MAX_POINTS_X
 
   #define UBL_MESH_EDIT_MOVES_Z     // Sophisticated users prefer no movement of nozzle

config/examples/Alfawise/U20-bltouch/Configuration_adv.h

 
 #if EITHER(MESH_BED_LEVELING, AUTO_BED_LEVELING_UBL)
   // Override the mesh area if the automatic (max) area is too large
-  //#define MESH_MIN_X MESH_INSET
-  //#define MESH_MIN_Y MESH_INSET
-  //#define MESH_MAX_X X_BED_SIZE - (MESH_INSET)
-  //#define MESH_MAX_Y Y_BED_SIZE - (MESH_INSET)
+  #define MESH_MIN_X _MAX(MESH_INSET, MIN_PROBE_EDGE)
+  #define MESH_MIN_Y _MAX(MESH_INSET, MIN_PROBE_EDGE)
+  #define MESH_MAX_X X_BED_SIZE - 35 // NOZZLE_TO_PROBE_OFFSET
+  #define MESH_MAX_Y Y_BED_SIZE - _MAX(MESH_INSET, MIN_PROBE_EDGE)
 #endif
 
 /**