[Hybrid_11]

I am not entirely sure what calculations your referring to. The only thing that pops in my head is counting outs and therefore the relative chance to win.
The concept used to determine if drawing is positive ev is done by counting all outs you believe to have. The most simple would be a flopped 4 flush which puts you on a flush draw. That would leave 9 remaining cards to complete the flush theoretically. Take that number and times it by 4 and that gives you a pretty good aproximation percentage of hitting the flush on either the turn or river combined. To find the odds of hitting on the river if you missed the turn is taking the outs and multiplying by 2. Once in percentage you can calculate that in odds
So with the flush draw
On the flop : theres 9x4=36 percent to hit by the river
On the turn : 9x2=18% to hit on the river

I hope thats what you are looking for if not please be more specific