dont get into the habit of calculating odds ...to the river... in a NL game. you cannot accurately estimate the turn bet, and that ruins the odds you just calculated. how do you know your villain wont check the turn? or bet the pot? or 1/2 the pot? all those change your pot odds on the turn, and you havent even gotten to that point yet.

easier to take your odds...to the next card...twice. recalculating each time.

flush is 9 outs...on the flop...you have 18-20% chance to hit on next card. you need 4:1. you miss...on the turn...you still have 18-20% to hit on next card. you still need 4:1. whatever miniscule percentage you miscalculate between "to the turn" and "to the river" calculations, you will make up for in implied odds anyway, imo. just easier that way.

in limit, you can do a better job at guessing/controlling the betting, and can therefore calculate "to the river" with more accuracy.