First, the taxpayer shall ascertain the percentage which the average net book value, of the tangible personal property owned and the real property, including leasehold improvements, owned or used by it in the business and situated within the city during the taxable period, is of the average net book value of all of such property, including leasehold improvements, owned or used by the taxpayer in the business during the same period wherever situated. Real property shall include real property rented or leased by the taxpayer and the value of such property shall be deemed to be 8 times the annual gross rental thereon. "Gross rental of real property" means the actual sum of money or other consideration payable, directly or indirectly, by the taxpayer for the use or possession of real property and includes but is not limited to:

    (a) An amount payable for the use or possession of real property or any part thereof, whether designated as a fixed sum of money or as a percentage of sales, profits or otherwise.

    (b) An amount payable as additional rent or in lieu of rent such as interest, taxes, insurance, repairs or other amount required to be paid by the terms of a lease or other arrangement.