Fix probing margin sanity-check

Marlin/src/inc/SanityCheck.h

     static_assert(PROBING_MARGIN_RIGHT >= 0, "PROBING_MARGIN_RIGHT must be >= 0.");
   #endif
 
-  #define _MARGIN(A) TERN(IS_SCARA, SCARA_PRINTABLE_RADIUS, TERN(DELTA, DELTA_PRINTABLE_RADIUS, A##_CENTER))
+  #define _MARGIN(A) TERN(IS_SCARA, SCARA_PRINTABLE_RADIUS, TERN(DELTA, DELTA_PRINTABLE_RADIUS, ((A##_BED_SIZE) / 2)))
   static_assert(PROBING_MARGIN < _MARGIN(X), "PROBING_MARGIN is too large.");
   static_assert(PROBING_MARGIN_BACK < _MARGIN(Y), "PROBING_MARGIN_BACK is too large.");
   static_assert(PROBING_MARGIN_FRONT < _MARGIN(Y), "PROBING_MARGIN_FRONT is too large.");