I do it the easy way...

Say we're on classic flush draw, 35%

If my bet is greater than 1/3 of the pot I fold.

If you want it described mathematically:
If Odds (.35) * Pot < Bet, fold.

You can also do:
If Pot < 3 * Bet, fold
in this situation for a faster estimate.

Also, you can do:
If Pot/Bet < 1/Odds, fold
or
If (Odds * Pot)/Bet < 1, fold.