Floor area ratio for office buildings. The total gross floor area of all principal office and accessory buildings and structures, excluding parking structures hereafter erected or structurally altered in a PCO District, shall not exceed 200% of the total land area that is occupied. In order to promote more open space, the allowable floor area ratio may be increased by 1% for each foot of each yard beyond the minimum required dimension, provided that the total increase so accrued shall not exceed 50%. Gross floor area is the sum of the areas of each floor.