**How do I calculate my hand odds flop to river?**

Say, someone goes all-in on the flop and you have a flush draw.

Odds from flop to turn are 4:1, yet they turn to 2:1 when going in on the flop to river, so how exactly are they calculated? Is it exactly the same thing as the rule of 2 and 4?

If I had a gutshot for example, then my hand odds are 10.75:1 on the flop and 10.5:1 on the turn. The average of the previous two is 10.6, so by that my chances double and now my odds are 10.6:2 or 5.3:1? I'm sorry if this is a dumb question, but I just wasn't sure.

Thanks for taking your time