29 U.S.C. § 1386 — Adjustment for partial withdrawal; determination of amount; reduction for partial withdrawal liability; procedures applicable

(a) Amount. Before applying sections 1399(c)(1) and 1405, partial-withdrawal liability equals:
(1) the amount determined under section 1391, adjusted under section 1389 when appropriate, as if the employer had completely withdrawn—on the partial-withdrawal date, or, for a 70-percent contribution decline under section 1385(a)(1), on the last day of the first plan year^* in the three-year testing period—multiplied by
(2) a fraction equal to 1 minus a second fraction. The second fraction’s numerator is the employer’s contribution-base units for the plan year after the partial withdrawal. Its denominator is the average units for the five plan years before the withdrawal, except that for a 70-percent decline it is the average for the five years before the testing period began.

(b) Later liability.
(1) If an employer has partial-withdrawal liability, its later partial- or complete-withdrawal liability from the same plan must be reduced by the earlier partial-withdrawal liability, after any abatement or reduction.
(2) The corporation must issue regulations to adjust that reduction for changes in unfunded vested benefits^* after the prior determination, changes in contribution-base units after that year, and other appropriate factors, so the later liability properly reflects the employer’s share.