the chance of getting a flush draw are (9 outs as 4 have been drawn so the remaining 9 cards of that suit multiply by 4 on the turn (9 X 4 = 36)) 36% thats a quick way of doing the math its closer to 34% if done correctly. but anyway thats on average 1 in 3 times you hit the flush, so you lose 2 out of every 3 making chasing it less than worth while.

but mostly it depends if your getting the correct odds to call as well, say 300 is in the pot and your opponent splashes 100, making the total pot 400 it will cost you 100 to call, brake this down we get 4 to 1 pot odds if we add the 4 and 1 (5) and divide by 100 (percentage) you get 20%. thus he is giving you the right pot odds to call by mathmatically saying you need a hand 20% or better to call with and since your is 36% you would be correct to call.

say your miss the turn and looking blankly now with still 9 outs with the river to go. more maths is needed we now still got 9 out but now multiply by 2 so we now have only 18% to make our flush draw. say our oppenent puts another 100 into the pot. the pot is now 600. making your call 6 to 1. now we do our adding then divide by 100 (100/7=14.3). so with 14.3% needed to call youve got roughly 18% so the correct decission would be to call (even though both on the turn and river a raise would also be a correct play). so play your pot odds and your out percentages and you'll do well

e.g. of a bad call.................. on the river on flush draw and the pot is 400, the opponent bets 400 bringing it to 800 with 400 to call making it (800/400 = 2/1 = 2+1 = 3 = 100/3 = 33.3333%) you need a hand 33% or better to make the call which we now know from earlier you only have a 18% chance on the river meaning we need to fold this hand and wait for a better chance)

now with the straight draws i persume you mean open ended which gives you 8 outs so on the turn for a quick percentage is 32% (1 in 3) and on the river only 16% (1 in 6)

if your after the gut shot straight draw youve got 4 outs so 16% chance on the turn (1 in 6) and only 8% on the river (1 in 12 times of hitting it)

hope this helps with you solving how to go for the draws