Outside public way. Where a boundary is shown outside of a public way, private way, railroad, utility transmission line, utility pipeline, natural watercourse and approximately parallel thereto, it shall be deemed parallel to the nearest exterior line thereof, and a figure placed on the Zoning Map between the boundary and the way, railroad, utility lines, or watercourse is the distance in feet of such boundary from such exterior line, said distance being measured at a right angle thereto unless otherwise indicated.