This is a non-intuitive change.   @oldmcg says in this post https://github.com/MarlinFirmware/Marlin/issues/7572#issuecomment-340641207 that using DELTA_PRINTABLE_RADIUS is better than DELTA_PROBABLE_RADIUS for setting the probe limits.   This is because the automatic probe code checks all locations for being reachable by the probe prior to doing the probe.