The boundaries are, unless otherwise indicated upon the Building Zone Map, either street lines or lines drawn parallel to and one hundred fifty (150') feet back from one or more of the street lines bounding the block. Where two or more designations are shown within a block three hundred (300') feet or less in width, the boundary of the more restricted zone shall be deemed one hundred fifty (150') feet back from the street line, and where two or more zone designations are shown within a block more than three hundred (300') feet in width, the boundary of the less restricted zone shall be deemed one hundred fifty (150') feet back from the street line.

A lot situated in the interior of a block and divided by the zone boundary line into two different zones may, insofar as it is situated at least one hundred fifty (150') feet from any street bounding the block in which it is located, be used in accordance with the use regulations of the less restricted zone into which it is divided.