Shopping centers: four parking spaces for each 1,000 square feet of gross floor area for centers having less than 400,000 square feet. Shopping centers having 400,000 square feet or more shall provide parking at the rate of 4.5 spaces for each 1,000 square feet of gross floor area. If more than 5% of the enclosed gross floor area of any shopping center is occupied by malls, lobbies, corridors, heating plants or other space not utilized for direct commercial purposes which, in the opinion of the Planning Board, will not generate a need for parking, the Planning Board shall allow the parking required to be based on the gross leasable area of the shopping center at the above rate. The "gross leasable area," for the purposes of this chapter, shall be defined as the total floor area designed for tenant occupancy and exclusive use, including basements, mezzanines and upper floors. See parking standard for theatres.