In determining the number of parking spaces required for a particular building site in a business district, if the building is 10,000 square feet or less and if the entity using the site has different types of uses which are separable and are significant in their proportion of the total use and such uses have different parking ratios established by generic usage in the Table of Parking Requirements, the parking ratio for the entire site may be calculated by apportioning the site usages to each particular use.