fix ubl bed inset z position

Marlin/src/feature/bedlevel/ubl/ubl.h

@@ -124,14 +124,14 @@ class unified_bed_leveling {
 
     static int8_t cell_index_x(const float &x) {
       const int8_t cx = (x - (MESH_MIN_X)) * RECIPROCAL(MESH_X_DIST);
-      return constrain(cx, 0, (GRID_MAX_POINTS_X) - 1);   // -1 is appropriate if we want all movement to the X_MAX
+      return constrain(cx, 0, (GRID_MAX_POINTS_X) - 2);   // -1 is appropriate if we want all movement to the X_MAX
     }                                                     // position. But with this defined this way, it is possible
                                                           // to extrapolate off of this point even further out. Probably
                                                           // that is OK because something else should be keeping that from
                                                           // happening and should not be worried about at this level.
     static int8_t cell_index_y(const float &y) {
       const int8_t cy = (y - (MESH_MIN_Y)) * RECIPROCAL(MESH_Y_DIST);
-      return constrain(cy, 0, (GRID_MAX_POINTS_Y) - 1);   // -1 is appropriate if we want all movement to the Y_MAX
+      return constrain(cy, 0, (GRID_MAX_POINTS_Y) - 2);   // -1 is appropriate if we want all movement to the Y_MAX
     }                                                     // position. But with this defined this way, it is possible
                                                           // to extrapolate off of this point even further out. Probably
                                                           // that is OK because something else should be keeping that from

Marlin/src/feature/bedlevel/ubl/ubl_motion.cpp

@@ -74,21 +74,13 @@
       FINAL_MOVE:
 
       // The distance is always MESH_X_DIST so multiply by the constant reciprocal.
-      const float xratio = (end.x - mesh_index_to_xpos(iend.x)) * RECIPROCAL(MESH_X_DIST);
-
-      float z1, z2;
-      if (iend.x >= GRID_MAX_POINTS_X - 1)
-        z1 = z2 = 0.0;
-      else {
-        z1 = z_values[iend.x    ][iend.y    ] + xratio *
-            (z_values[iend.x + 1][iend.y    ] - z_values[iend.x][iend.y    ]),
-        z2 = z_values[iend.x    ][iend.y + 1] + xratio *
-            (z_values[iend.x + 1][iend.y + 1] - z_values[iend.x][iend.y + 1]);
-      }
+      const float xratio = (end.x - mesh_index_to_xpos(iend.x)) * RECIPROCAL(MESH_X_DIST),
+                  yratio = (end.y - mesh_index_to_ypos(iend.y)) * RECIPROCAL(MESH_Y_DIST),
+                  z1 = z_values[iend.x][iend.y    ] + xratio * (z_values[iend.x + 1][iend.y    ] - z_values[iend.x][iend.y    ]),
+                  z2 = z_values[iend.x][iend.y + 1] + xratio * (z_values[iend.x + 1][iend.y + 1] - z_values[iend.x][iend.y + 1]);
 
       // X cell-fraction done. Interpolate the two Z offsets with the Y fraction for the final Z offset.
-      const float yratio = (end.y - mesh_index_to_ypos(iend.y)) * RECIPROCAL(MESH_Y_DIST),
-                  z0 = iend.y < GRID_MAX_POINTS_Y - 1 ? (z1 + (z2 - z1) * yratio) * planner.fade_scaling_factor_for_z(end.z) : 0.0;
+      const float z0 = (z1 + (z2 - z1) * yratio) * planner.fade_scaling_factor_for_z(end.z);
 
       // Undefined parts of the Mesh in z_values[][] are NAN.
       // Replace NAN corrections with 0.0 to prevent NAN propagation.