Assessments shall be levied according to a formula which shall be set forth in the plan and which shall be reasonable and proportional to the benefits conferred upon the land or leasehold interest upon which such assessment is laid. Such formula may, but need not necessarily, be based on the number of off-street parking spaces required to be furnished by owners of land under any lawful zoning ordinances or bylaw in effect or which may be adopted by the municipality. If the formula based on a zoning ordinance requirement of off-street parking is used, the plan may provide for credit to those owners or lessees who have erected or constructed private parking structures, but need not provide credit for private parking lots.