(a) Tax previously determined. The taxpayer is to determine the amount of decrease, attributable to the carryback, in tax previously determined for each taxable year before the taxable year of the net operating loss, net capital loss, or unused investment credit. The tax previously determined is to be ascertained in accordance with the method prescribed in section 1314(a). Thus, the tax previously determined will be the tax shown on the return as filed, increased by any amounts assessed (or collected without assessment) as deficiencies before the date of the filing of the application for a tentative carryback adjustment, and decreased by any amounts abated, credited, refunded, or otherwise repaid prior to that date. Any items as to which the Commissioner and the taxpayer are in disagreement at the time of the filing of the application shall, for purposes of § 1.6411-2, be taken into account in ascertaining the tax previously determined only if, and to the extent that, they were reported on the return, or were reflected in any amounts assessed (or collected without assessment) as deficiencies, or in any amounts abated, credited, refunded, or otherwise repaid, before the date of filing the application. The tax previously determined, therefore, will reflect the foreign tax credit and the credit for tax withheld at source provided in section 33.

(b) Decrease attributable to carryback. After ascertaining the tax previously determined in the manner described in paragraph (a) of this section, the taxpayer shall determine the decrease in tax previously determined attributable to the carryback and any related adjustments on the basis of the items of tax taken into account in computing the tax previously determined. In determining any decrease attributable to the carryback or any related adjustment, items shall be taken into account under this subsection only to the extent that they were reported on the return, or were reflected in amounts assessed (or collected without assessment) as deficiencies, or in amounts abated, credited, refunded, or otherwise repaid, before the date of filing the application for a tentative carryback adjustment. If the Commissioner and the taxpayer are in disagreement as to the proper treatment of any item, it shall be assumed, for purposes of determining the decrease in the tax previously determined, that the item was reported correctly by the taxpayer unless, and to the extent that, the disagreement has resulted in the assessment of a deficiency (or the collection of an amount without an assessment), or the allowing or making of an abatement, credit, refund, or other repayment, before the date of filing the application. Thus, if the taxpayer claimed a deduction on its return of $50,000 for salaries paid its officers but the Commissioner proposes that the deduction should not exceed $20,000, and the Commissioner and the taxpayer have not agreed on the amount properly deductible before the date the application for a tentative carryback adjustment is filed, $50,000 shall be considered as the amount properly deductible for purposes of determining the decrease in tax previously determined in respect of the application for a tentative carryback adjustment. In determining the decrease in tax previously determined, any items that are affected by the carryback must be adjusted to reflect the carryback. Thus, unless otherwise provided, any deduction limited, for example, by adjusted gross income, such as the deduction for medical, dental, etc., expenses, is to be recomputed on the basis of the adjusted gross income as affected by the carryback. See § 1.6411-3(d) for rules on the application of the decrease in tax to any tax liability.

(c) Effective/applicability date. These regulations apply with respect to applications for tentative refund filed on or after August 27, 2007.