(a) Tax previously determined. The taxpayer is to determine the amount of the reduction, attributable to the expected carryback, in the aggregate of the taxes previously determined for taxable years prior to the taxable year of the expected net operating loss. 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 prior to the date of the filing of the statement, and decreased by any amounts abated, credited, refunded, or otherwise repaid prior to such date. Any items as to which the Internal Revenue Service and the taxpayer are in disagreement at the time of the filing of the statement shall be taken into account in ascertaining the tax previously determined only if, and to the extent that, they were reported in 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, prior to the date of the filing of the statement. The tax previously determined will reflect the foreign tax credit and the credit for tax withheld at source provided in section 32.

(b) Reduction attributable to the expected carryback. The reduction, attributable to the expected carryback or related adjustments, in any tax previously determined is to be ascertained by applying the expected carryback as if it were a determined net operating loss carryback, in accordance with the provisions of section 172 and the regulations thereunder. Items must be taken into account only to the extent that such items were included in the return, or were reflected in amounts assessed (or collected without assessment) as deficiencies, or in amounts abated, credited, refunded, or otherwise repaid, prior to the date of the filing of the statement. Thus, for example, if the taxpayer claims a deduction for depreciation of $10,000 in its return and the Internal Revenue Service asserts that only $4,000 is properly deductible, no change is to be made in the $10,000 depreciation deduction as shown by the taxpayer on his return unless a deficiency has been assessed, or an amount collected without assessment, prior to the date of filing of the statement as a result of a change in the depreciation deduction, or unless such change in the depreciation deduction was reflected in an amount abated, credited, refunded, or otherwise repaid prior to such date.