Twenty percent of the total number of units constructed within the MDR Zone shall be affordable to low- and moderate-income households. When calculating the required number of affordable units, any computation resulting in a fraction of less than 0.5 shall be rounded down; any computation resulting in a fraction of 0.5 or greater shall be rounded up; provided, however, that a minimum of one affordable unit shall be required as part of any project constructed within the MDR Zone.