Pot odds and flushes calculation clarification
 

I was explaining pot odds and the effect on drawing hands to a friend today and I think I might not even completely understand it myself. Can someone please look over my reasoning/maths and just tell me if I'm wrong/right.

Assume there are not bets on the turn, and that you're drawing to the nut flush and ignore implied odds.

Basically if you hold 2 hearts and there are two hearts on the flop you have aproximately a 31.5% (not sure where I got this from but I'm pretty sure this is right, or using the rule of fours you could say theres a 36%) chance that you will hit your flush by the river.

So basically you need pot odds of at least 1:2.2 (1/3.2 is approx. 31.5) which means that if a pot is $12 you need to be calling <10. If, however, you don't want to give your opponent the correct odds to call you need to be betting >10.

Is this right? What confused me mostly is, if my above reasoning is right, then if I used the rule of fours to estimate the odds, getting a 36% chance of hitting a flush then the odds to call would be 1:1.8, which means if a pot is $8 I can profitably call a $10 bet and still draw to a flush. However, I'm sure this is wrong.

So the first way I described correct and the one that I should follow?