(a) General rule. A credit of the applicable credit amount shall be allowed to the estate of every decedent against the tax imposed by section 2001 [26 USCS § 2001].
(b) Adjustment to credit for certain gifts made before 1977. The amount of the credit allowable under subsection (a) shall be reduced by an amount equal to 20 percent of the aggregate amount allowed as a specific exemption under section 2521 [26 USCS § 2521] (as in effect before its repeal by the Tax Reform Act of 1976) with respect to gifts made by the decedent after September 8, 1976.
(c) Applicable credit amount.
(1) In general. For purposes of this section, the applicable credit amount is the amount of the tentative tax which would be determined under section 2001(c) if the amount with respect to which such tentative tax is to be computed were equal to the applicable exclusion amount.