(a) Limitation. No application under section 6425 shall be allowed unless the amount of the adjustment is (1) at least 10 percent of the amount which, at the time of filing Form 4466 the corporation estimates as its income tax liability for the taxable year, and (2) at least $500.

(b) Time prescribed. The Internal Revenue Service shall act upon an application for an adjustment of overpayment of estimated income tax within a period of 45 days from the date on which such application is filed.

(c) Examination. Within the 45-day period described in paragraph (b) of this section, the Internal Revenue Service shall make, to the extent it deems practicable in such period, a limited examination of the application to discover omissions and errors therein. The Service shall calculate the adjustment, which calculation must be set forth in the application for such adjustment, in the manner provided in section 6425(c)(2) for the determination by the corporation of such adjustment. The Service, however, may correct any material error or omission that is discovered upon examination of the application. In determining the adjustment, the Service may correct any mathematical error appearing on the application, and it may likewise make any modification required by the law to correct the corporation's computation of the adjustment. If the required modification has not been made by the corporation and the Service has available the necessary information to make such modification within the 45- day period, it may make such modification. The examination of the application and the allowance of the adjustment shall not prejudice any right of the Service to claim later that the adjustment was improper.

(d) Disallowance in whole or in part. If the Internal Revenue Service finds that an application for an adjustment of overpayment of estimated tax contains material omissions or errors, the Service may disallow such application in whole or in part without further action. If, however, the Service deems that any omission or error can be corrected by it within the 45-day period, it may do so and allow the application in whole or in part. In the case of a disallowance or modification, the Service shall notify the corporation of such action. The Service's determination as to whether it can correct any omission or error shall be conclusive. Similarly, its action in disallowing, in whole or in part, any application for an adjustment of overpayment of estimated income tax shall be final and may not be challenged in any proceeding. The corporation in such case, however, may file a claim for credit or refund under section 6402, and may maintain a suit based on such claim if it is disallowed or if the Service does not act upon the claim within 6 months from the date it is filed.

(e) Application of adjustment. If the Internal Revenue Service allows the adjustment, it may first credit the amount of the adjustment against any liability in respect of an internal revenue tax on the part of the corporation which is due and payable on the date of the allowance of the adjustment before making payment of the balance to the corporation. In such a case, the Service shall notify the corporation of the credit, and refund the balance of the adjustment.

(f) Effect of adjustment. (1) For purposes of all sections of the Internal Revenue Code except section 6655, relating to additions to tax for failure to pay estimated income tax, any adjustment under section 6425 is to be treated as a reduction of prior estimated tax payments as of the date the credit is allowed or the refund is paid. For the purpose of sections 6655(a) through (g), (i), and (j), credit or refund of an adjustment is to be treated as if not made in determining whether there has been any underpayment of estimated income tax and, if there is an underpayment, the period during which the underpayment existed. However, an excessive adjustment under section 6425 is taken into account in applying the addition to tax under section 6655(h).

(2) For the effect of an excessive adjustment under section 6425, see § 1.6655-7.

(3) Effective/applicability date: This paragraph (f) is applicable to applications for adjustments of overpayments of estimated income tax that are filed in taxable years beginning after September 6, 2007.