Well, you would want the probability of hitting your straight with no flush cards coming. Given that you KNOW someone is drawing to a flush (therefore has 2 of that suit) there are nine cards of the flush suit left and 36 non-flush cards. You have 6 non-flush suit cards with which to make your straight. So there are 6*35 two card combinations which make the straight and not the flush. There are 45C2 =990 total 2 card combinations left. So you will make your straight with no flush about 21% of the time, or you are about a 3.7:1 dog to make straight without flush coming. Note: this does not take into account the board pairing and being up against a full house, or one of your straight cards being counterfeited and losing to a higher straight.