The floor area ratio regulations of the underlying districts are modified in accordance with the provisions of Section 81-21, inclusive, or Section 81-241 (Maximum floor area ratios for a residential building or the residential portion of a mixed building). However, the provisions of Section 81-21, inclusive, shall not apply to non-residential buildings or mixed buildings in the East Midtown Subdistrict, where the special floor area provisions of Sections 81-63 or 81-64 shall apply.