Whenever a zone boundary line passes through a lot of record which existed prior to the passage of this title the entire lot of record may be used in accordance with the provisions of the least restrictive of the two zones; provided, that more than 50 percent of the lot of record is located within the least restrictive of the two zones. For the purposes of boundary interpretation the zones shall be classified in terms of restriction as follows (least restrictive to most restrictive): M-II, M-I, MU, C-III, C-II, C-I, R-III, R4-8, R-I(S), R-I(LS), PF, B.