Designation of district boundaries: Boundaries indicated as approximately following the center line of streets, highways, alleys, railroad rights-of-way, streams, watercourses, existing lot lines or municipal boundary lines shall be construed as to follow such features indicated. Where a district boundary line does not follow such a feature, its position is indicated 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.