Wherever any uncertainty exists as to the boundary of a use district as shown on the Zoning Map, the following rules shall apply:
(a) Where district boundary lines are indicated as following streets, alleys or similar rights of way, they shall be construed as following the centerlines thereof.
(b) Where district boundary lines are indicated as approximately following lot lines, such lot lines shall be construed to be such boundaries.
(c) Where a lot held in one ownership and of record on the effective date of this Zoning Code is divided by a district boundary line, the entire lot shall be construed to be within the less restricted district. However, this construction shall not apply if it increases the area of the less restricted portion of the lot by more than twenty percent.