1. Yes... go look over the examples I posted with actual numbers and see for yourself WHY you should call
2. A flush isn't 31% first of all...
but ignoring that, to keep it simple it takes the number of cards that will help you (in a flush draw case 9 cards will help you, since there are 9 remaining diamonds, or whatever suit you have left in the deck) and compares it to the number of unseen cards. There are 52 cards in a deck, you know what 2 of them are (from whats in your hand) and what 3 more of them are (the flop) That means there are 47 'unknown cards'. NOW I realize that there arent 47 cards left in the deck, because your opponents have some of them... but since we do not know what cards your opponents have, we have to include them in the unseen cards. Now if we knew that your 4 opponents had 2 cards each and none was a diamond, then there'd be 39 unknown cards instead and the % would go up... but since we don't know we have to assume there is an equal chance of there being diamonds in your opponents hands as there is for them not to have diamonds.
So 47 cards remaining, which means that 9 will help you and 38 will not help you
38 / 9 = 4.2
Now say you miss on the turn.
Now there are 46 unknown cards (because we know one more, the turn card) and still 9 cards that will help us (the 9 diamonds still) but now only 37 that will not help us
37 / 9 = 4.1
So these are roughly your chances that you will hit on JUST the turn or JUST the river (roughly 4:1)
But, you have 9 outs twice ( once on the turn and again on the river)
I used to just say thats a total of 18 outs (9+9) and 27 non outs 29:9 = 38%, but that's off by 2% and obvious I can't figure it that way since I don't actually have 18 outs and the number of cards changes on the turn and river... rather than figure out the exact way to it it, I just memorized an odds chart and stuck with it
(a way of saying ask someone who is better at math than I am) lol