you assume, after you have processed the initial stage, that the titration is zero (ie you assume that all the FFAs have been neutralised)
in laymans terms, you just use the base value for the second stage. most would consider that to be 7g ltr for koh
so, 2.1ml fall out from the 10/90 would be 21%
21% of your 130ltr batch would be 130 x 0.21 = 27.3 ltrs unconverted.
so 191.1 g koh