(a) The accounting method or basis on which entire net income is to be computed must be the same as the taxpayer's method of accounting for Federal income tax purposes. However, when the Commissioner of Taxation and Finance deems it necessary in order to properly reflect the entire net income of the taxpayer, it may determine the taxable year or period in which any item of income or deduction must be included, without regard to the method of accounting used by the taxpayer. (See section 3-2.7 of this Title—Taxable year in which income or deduction is included in entire net income.)

(b) In the absence of an accounting method for Federal income tax purposes, entire net income must be computed in accordance with the method regularly employed in keeping the books of the taxpayer, provided such method properly reflects entire net income. If the books of a taxpayer do not properly reflect entire net income, or if no books are kept, the computation of entire net income must be made in such manner as the Commissioner of Taxation and Finance deems necessary to properly reflect entire net income.