Parking spaces with 25 feet of rear maneuvering space, shall be provided as follows: one space for each 1.5 employees, computed on the basis of combined employment of the largest and second largest shifts; or one space for each 3,000 square feet of gross floor area used for warehousing; or one space for each 1,000 square feet of gross floor area used for manufacturing, plus one space for each company-owned truck or vehicle, plus one visitor parking space for each 1,000 square feet of office space.