(a) Federal income taxes, state and local income taxes, and state and local gross receipts or gross earnings taxes that are collected in lieu of a corporate income tax shall be apportioned among the interexchange category, the billing and collection category and all access elements based on the approximate net taxable income on which the tax is levied (positive or negative) applicable to each element and category.

(b) All other operating taxes shall be apportioned among the interexchange category, the billing and collection category and all access elements in the same manner as the investment apportioned to each element and category pursuant to §69.309 Other Investment.