For zoning lots divided by district boundaries in which all applicable height and setback regulations include the use of sky exposure planes, the height and setback regulations of each street frontage of the zoning lot shall be determined by multiplying the quantitative requirements set forth in the regulations of the Chapters, which are applicable to each portion of such street frontage, by the percentage of such street frontage to which such regulations apply. The sum of the products obtained shall be the controlling requirements for the zoning lot.

In determining the percentage of such street frontage, the percentage shall be based on the total frontage of the zoning lot along such street.

However, if any portion of such zoning lot is located within a Limited Height District, the provisions of Sections 23-691, 24-591, 33-491 or 43-49 (Limited Height Districts) shall apply to such portion of the zoning lot.

For all other zoning lots, each portion of such zoning lot shall be regulated by the height and setback provisions applicable to the district in which such portion of the zoning lot is located.

In R2X, R3, R4 or R5 Districts, for residential portions of buildings, each portion of the zoning lot shall be governed by the height and setback regulations specified for the district in which it is located, as set forth in Article II, Chapter 3.

For the purposes of defining a building envelope pursuant to

paragraph (b) of Section 23-631 (General provisions), apex points may be located on a zoning district boundary which divides a building.

Furthermore, if any portion of a zoning lot is located in an R2X, R3, R4, R4-1 or R4A District, the height and setback regulations specified for such district may apply to the entire zoning lot provided that such district comprises more than 50 percent of such zoning lot, and the greatest distance from the mapped district boundary to any lot line of such zoning lot in the district in which less than 50 percent of its area is located does not exceed 25 feet. Such distance shall be measured perpendicular to the mapped district boundary.