For zoning districts in certain Special Mixed Use Districts specified by this Section, the height and setback regulations of paragraphs (a) and (b) of Section 123-662 (All buildings in Special Mixed Use Districts with R6, R7, R8, R9 and R10 District designations) shall be modified as follows:
 

1. In R8X Districts within Special Mixed Use District 2:
   1. the maximum building height for any development or enlargement that is not predominantly commercial, manufacturing, or a combination of commercial and manufacturing, shall be governed by the underlying maximum building height and setback regulations applicable to an R8A District. For the purposes of this provision, “predominantly” shall mean at least 75 percent of the floor area of a building; and
       
   2. in Historic Districts designated by the Landmarks Preservation Commission, the maximum base height of a street wall may vary between the maximum base height of this Section, and the height of the street wall of an adjacent building before setback, if such height is higher than such maximum base height. For the purposes of this paragraph, a building situated directly across a street from a development shall be considered an adjacent building.
       
2. In R7-2 Districts within Special Mixed Use District 15:
   1. a building or other structure, or portion thereof, located within 10 feet of a wide street or 15 feet of a narrow street, shall rise to a minimum height of 60 feet, and may rise to a maximum height of 85 feet;
       
   2. at least 70 percent of the aggregate width of street walls shall be located on the street line and shall extend to the minimum base height of 60 feet or the height of the building, whichever is less. The remaining 30 percent of the aggregate width of street walls shall be located within eight feet of the street line; and
       
   3. existing buildings may be vertically enlarged by up to one story or 15 feet without regard to the street wall location provisions of this paragraph. Beyond 10 feet of a wide street and 15 feet of a narrow street, the height of a building or other structure shall not exceed a maximum building height of 135 feet. However, a building or other structure may exceed a height of 135 feet by four stories or 40 feet, whichever is less, provided that the gross area of each story located above 135 feet does not exceed 80 percent of the gross area of that story directly below it.
       
3. In R6A Districts within Special Mixed Use District 23, where such R6A District is paired with an M1-5 District, the underlying height and setback regulations applicable to an R7A District shall apply.