(a) The amount of an employer’s liability for a partial withdrawal, before the application of sections 1399(c)(1) and 1405 of this title, is equal to the product of—
(1) the amount determined under section 1391 of this title, and adjusted under section 1389 of this title if appropriate, determined as if the employer had withdrawn from the plan in a complete withdrawal—
(2) a fraction which is 1 minus a fraction—
(B) the denominator of which is the average of the employer’s contribution base units for—
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.
(2) The corporation shall prescribe such regulations as may be necessary to provide for proper adjustments in the reduction provided by paragraph (1) for—
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