| Phil Gordon talks about the same approximation. Multiply your outs * 4 and that gives you your percentage when there are 2 cards to come. Multiply by 2 with one card to come, as you said. It seems to be good enough to use as an approximation. In most cases, you don't need to know the numbers down to 2 decimals anyway, so whatever makes it easier for you.
It's trickier in no limit because of implied odds. Also, to correct what you said, if you're getting 4 to 1 after the turn on a flush draw, you do have the correct odds to call without even considering implied odds. After the flop is a little different. Technically, you are 2 to 1 against, not 4-1, hitting your flush draw in the next 2 cards. However, if you are playing an aware opponent in no limit, you can't guarantee you'll get to see the river for the correct odds. So in no limit, you can almost treat the turn like the river. If you're getting 4 to 1 to see the turn, you're getting correct odds. Less than 4-1 and you have to start considering implied odds and all sorts of other things like chip stacks, etc.  | 
