In C1 or C2 Districts mapped within R9 and R10 Districts, or in Commercial Districts with a residential equivalent of an R9 or R10 District, for zoning lots containing a building that is developed or enlarged pursuant to the applicable tower regulations of Section 35-64 (Special Tower Regulations for Mixed Buildings), the provisions of paragraph (a)(2) of Section 23-16 (Special Floor Area and Lot Coverage Provisions for Certain Areas) shall apply:
(a) to only the residential portion of a building where less than 75 percent of the total floor area of such building is allocated to residential use; and
(b) to the entire building where 75 percent or more of the total floor area of such building is allocated to residential use.