Where a district boundary line, as established in this chapter or as shown on the Zoning Map, divides a lot or lots in common ownership and of record at the time of enactment of this chapter the least restrictive use shall be considered as extending to the entire lot, if the more restrictive portion of such lot is entirely within 25 feet or less of said dividing district boundary line. The use so extended shall be deemed to be conforming. If the more restrictive portion of the lot is not entirely within 25 feet of said dividing district boundary line, the various portions of the lot shall be zoned according to the underlying zone district classifications.