(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1391 of this title, and adjusted under section 1389 of this title if appropriate, determined as if the employer had withdrawn from the plan in a complete withdrawal—
on the date of the partial withdrawal, or
(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—
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 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—
so that the liability for any complete or partial withdrawal in any subsequent year (after the application of the reduction) properly reflects the employer’s share of liability with respect to the plan.