Deck Penetration And Card Counting

Ok-I found a pretty good double deck game: DOA,DAS,S17, so I figured the house edge to be about -.25%. Using the KO count I figured a 1-5 spread should be able to beat it. HOWEVER, the penetration is lousy in this game-usually around 50% alhough some dealers seem to give a little more-say 60-70%. My question is since the penetration is so lousy I'm beginning to think I'll probably need a bigger betting spread right? (to overcome the house edge since it is going to be harder for this game to go into positive count territory.) But how to calculate that? Is there a formula to follow that gives the right betting spread based on deck penetration?