Maximum residential gross density shall be eight units per acre, exclusive of any concierge residential units in the 13 story tower building, but subject to the following enhanced bonus density: 12 units per acre, provided as one residential unit for each 333 square feet of first floor retail and personal service spaces for the uses set forth in subsection 21-32.1a2(a) (Qualifying Main Street Commercial) only are constructed up to the maximum bonus density of 12 units per acre. By way of example, if 100,000 square feet of Qualifying Main Street Commercial space is constructed, 300 bonus residential units may be constructed.