(a) Income tax liability defined. For purposes of § 1.6425-1, this section, §§ 1.6425-3 and 1.6655-7, relating to excessive adjustment, the term income tax liability means the excess of—

(1) The sum of—

(i) The tax imposed by section 11 or 1201(a), or subchapter L of chapter 1 of the Internal Revenue Code, whichever is applicable; plus

(ii) The tax imposed by section 55; over

(2) The credits against tax provided by part IV of subchapter A of chapter 1 of the Internal Revenue Code.

(b) Computation of adjustment. The amount of an adjustment under section 6425 is an amount equal to the excess of the estimated income tax paid by the corporation during the taxable year over the amount which, at the time of filing Form 4466, the corporation estimates as its income tax liability for the taxable year.

(c) Effective/applicability date. Paragraph (a) of this section is applicable to applications for adjustments of overpayments of estimated income tax that are filed in taxable years beginning after September 6, 2007.