(a) Total amount to which extension may relate. The total amount of tax the time for payment of which may be extended under section 6164 may not exceed the amount of the reduction of the taxes previously determined attributable to the expected carryback.

(b) Amount of tax to which extension may relate. (1) The taxpayer shall specify on Form 1138 the kind of tax and the amount thereof the time for payment of which is to be extended. The amount of tax to which an extension may relate shall not exceed the amount of such tax shown on the return as filed, increased by any amount assessed as a deficiency (or as interest or addition to the tax) prior to the date of filing the statement and decreased by any amount paid or required to be paid prior to such date. In determining the amount of tax required to be paid prior to the date of filing the statement, only the following amounts shall be taken into consideration:

(i) The amount of the tax shown on the return as filed; and

(ii) Any amount assessed as a deficiency (or as interest or addition to the tax) if the tenth day after notice and demand for its payment occurs prior to the date of the filing of the statement.

(2) Delinquent installments are to be considered amounts required to be paid prior to the date of filing the statement. In the case of any authorized extension of time under sections 6161 and 6162, the amount of tax the time for payment of which is so extended is not to be considered required to be paid prior to the end of such extension. Similarly, any amount assessed as a deficiency (or as interest or addition to the tax) is not to be considered required to be paid prior to the date of the filing of the statement unless the tenth day after notice and demand for its payment falls prior to the date of the filing of the statement.

(3) The taxpayer may choose to extend the time for payment of all of one or more taxes, or it may choose to extend the time for payment of portions of several taxes. The taxes chosen by the taxpayer need not be those taxes which are affected by the carryback.