In R9 and R10 Districts, for zoning lots containing a building that is developed or enlarged pursuant to the applicable tower regulations of Section 23-435 (Tower regulations), any floor space used for mechanical equipment provided pursuant to paragraph (8) of the definition of floor area in Section 12-10 (DEFINITIONS), and any floor space that is or becomes unused or inaccessible within a building, pursuant to paragraph (k) of the definition of floor area in Section 12-10, shall be considered floor area and calculated in accordance with the provisions of this Section, provided that such floor space:

1. occupies the predominant portion of a story;
2. is located above the base plane or curb level, as applicable, and below the highest story containing residential floor area; and
3. exceeds an aggregate height of 25 feet within any given 75 vertical feet of one another within a building.

For the purpose of applying this provision, the height of such floor space shall be measured from the top of a structural floor to the bottom of a structural floor directly above such space. In addition, the number of stories of floor area such space constitutes within the building shall be determined by aggregating the total height of such floor spaces, dividing by 25 feet, and rounding to the nearest whole integer.