I think the way I did my calc. in previous problem counts a few of the hands twice (those with both cards coming from the 6 which give you a clean straight). To resolve this, I think I need to further break down remaining deck. 30 cards neither making straight or flush, 9 cards making flush (including suited cards that make your straight), 6 cards making only straight.


So there should be 6*30 = 180 combinations of one straight and one non-straight non-flush cards, 6C2 = 15 combinations of two straight cards. So probability of straight no flush is

(180 + 15)/990 = 19.6% or about 4:1 against. Sorry for any confusion.