Odds After the Flop

[RacersEdge]

Calculating pot odds relative to the odds of making a hand is pretty straightforward after the river card . My question is after the flop.

Let's say you have 4 to a flush after the flop. So at this point the odds of making the flush over the next 2 cards are about 2-1. The odds off making the flush on the rier are about 4-1.

Now at this point, strictly speaking, what pot odds are being compared to what hand odds to make the decision on playing?

I would always call - maybe raise in this particular scenario - but I got to thinking about the exact comaprisons here. Is it just your required bet on the flop relative to pot odds needs to be better than 2-1? It seems it would be more accurate to use your implied pot odds and your future bets to make the decision, because the 2-1 is relative to the the next 2 cards.

What's the exact math here?