(a) For purposes of this section, “gross receipts” means all revenues reportable by a taxable entity on its federal tax return, without deduction for the cost of property sold, materials used, labor performed, or other costs incurred, unless otherwise specifically provided in this chapter.
(b) Except as otherwise provided by this section, a taxable entity shall use the same accounting methods to apportion margin as used in computing margin.

(c) A taxable entity may not change its accounting methods used to calculate gross receipts more often than once every four years without the express written consent of the comptroller. A change in accounting methods is not justified solely because it results in a reduction of tax liability.