Designation of District Boundaries. Boundaries indicated as approximately following the center line of streets, highways, alleys, railroad rights-of-way, streams, existing lot lines or municipal boundary lines shall be construed to follow such features indicated. Where district boundary line does not follow such a line, position is shown on said zoning map by a specific dimension expressing its distance, in feet, from a street right-of-way line or other boundary line as indicated and running parallel thereto.