will 2 balls be enough?

ok, so...i have 2 balls of ksh and i'm sort of worried that i might run out. i have a 36" bust, but am planning on making one size smaller. also, i'm going to knit it in the round, which should use less yarn also.

do i have a shot or should i just suck it up and buy three new balls?*

*i purchased these 2 balls at a store-closing sale, so i'm pretty sure i won't be able to get the same dye lot.