(1) The withdrawal liability of an employer to a plan is the amount determined under section
1391 of this title to be the allocable amount of unfunded vested benefits, adjusted
(A) first, by any de minimis reduction applicable under section
1389 of this title,
(B) next, in the case of a partial withdrawal, in accordance with section
1386 of this title,
(C) then, to the extent necessary to reflect the limitation on annual payments under section
1399 (c)(1)(B) of this title, and
(D) finally, in accordance with section
1405 of this title.