Use [XY]_CENTER for error correction

Followup to #7512

Marlin/ubl_G29.cpp

       err_flag = true;
     }
 
-    if (!WITHIN(RAW_X_POSITION(g29_x_pos), X_MIN_BED, X_MAX_BED))  // if X & Y are not valid, use center of the bed values
-      g29_x_pos = (X_MIN_BED + X_MAX_BED) / 2.0;
-
-    if (!WITHIN(RAW_Y_POSITION(g29_y_pos), Y_MIN_BED, Y_MAX_BED))  // if X & Y are not valid, use center of the bed values
-      g29_y_pos = (Y_MIN_BED + Y_MAX_BED) / 2.0;
+    // If X or Y are not valid, use center of the bed values
+    if (!WITHIN(RAW_X_POSITION(g29_x_pos), X_MIN_BED, X_MAX_BED)) g29_x_pos = LOGICAL_X_POSITION(X_CENTER);
+    if (!WITHIN(RAW_Y_POSITION(g29_y_pos), Y_MIN_BED, Y_MAX_BED)) g29_y_pos = LOGICAL_Y_POSITION(Y_CENTER);
 
     if (err_flag) return UBL_ERR;