Calculating Implied Odds on the Fly

[HoldingFolding]

Has anyone come up with an easy calculable rule of thumb for whether to call a draw (or alternatively taking away the odds for calling a draw) in NLHE.

Example:
On a pure probability basis, no reads etc. There's x in the pot, after the flop two players remaining, he bets into you for y, you have a nut flush draw. We'll presume he will bet into you for a similar amount if the flush fails to materialise and that you'll be able to get a similar amount out of him if it does (big assumptions granted).

I need something I can calculate online i.e. You call if the number of outs divided by twenty, times the size of the pot is greater than the size of the raise.

[tek]

In NL your implied odds is potentially your remaining stack size times the number of players still in the hand.