(a) General. If an employer's withdrawal liability for a prior partial withdrawal has been reduced or waived, the credit determined pursuant to §§ 4206.4 through 4206.7 shall be adjusted in accordance with this section.

(b) Computation. The adjusted credit is calculated by multiplying the credit determined under the preceding sections of this part by a fraction—

(1) The numerator of which is the excess of the total partial withdrawal liability of the employer for all partial withdrawals in prior years (excluding those partial withdrawals for which the credit is zero) over the present value of each abatement or other reduction of that prior withdrawal liability calculated as of the date on which that prior partial withdrawal liability was determined; and

(2) The denominator of which is the total partial withdrawal liability of the employer for all partial withdrawals in prior years (excluding those partial withdrawals for which the credit is zero).