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

on the date of the partial withdrawal, or in the case of a partial withdrawal described in section 1385(a)(1) of this title (relating to 70-percent contribution decline), on the last day of the first plan year in the 3-year testing period, the numerator of which is the employer’s contribution base units for the plan year following the plan year in which the partial withdrawal occurs, and except as provided in clause (ii), the 5 plan years immediately preceding the plan year in which the partial withdrawal occurs, or in the case of a partial withdrawal described in section 1385(a)(1) of this title (relating to 70-percent contribution decline), the 5 plan years immediately preceding the beginning of the 3-year testing period. In the case of an employer that has withdrawal liability for a partial withdrawal from a plan, any withdrawal liability of that employer for a partial or complete withdrawal from that plan in a subsequent plan year shall be reduced by the amount of any partial withdrawal liability (reduced by any abatement or reduction of such liability) of the employer with respect to the plan for a previous plan year. changes in unfunded vested benefits arising after the close of the prior year for which partial withdrawal liability was determined, changes in contribution base units occurring after the close of the prior year for which partial withdrawal liability was determined, and any other factors for which it determines adjustment to be appropriate,