For all developments and enlargements on secondary transit-adjacent sites and conversions on all transit-adjacent sites seeking to provide a transit volume, an initial determination by the transit agency shall be made in accordance with the provisions of paragraph (a) of Section 66-21 (Determination and Certification for Transit Volumes). Where a transit volume is needed based on the initial determination, the dimensions and location of such transit volume shall be established in accordance with paragraph (b) of Section 66-21, and certified by the Chairperson of the City Planning Commission pursuant to paragraph (c) of such Section.