If you were to click the link at the bottom of this post, you would be taken to the NL Beginners course, where you could ask the coaches all sorts of questions about odds/outs/implied odds/reverse implied odds and anything else poker - and often non poker - related and it's totally free to join! Cool idea or what? Check it out and see for yourself.

I use something in between. For the next card it's (where X is our outs) X*2+2 for the next card and X*4 for flop and turn combined. That is usually accurate enough in playing situation.

Example

You have a flush draw, 9 outs. Chance to hit it on the next card is 9*2+2=20. That is precisely right. Chance to hit on flop or turn is 9*4=36 which is almost 35 which is the correct answer.