Ratio of: Total pot + your call + future bet to your call
Say you are getting 2:1 odds on a draw call. In order to break even you need 4:1 odds. Ask yourself if the player will likely bet more or less the value of chips on the following street to give you 4:1. This will give you the minimum calling implied odds based on villains future bet.
Example
Single raised pot, you cbet and get raised. Assume 100 stacks, and at flop decision you have about 25 in the pot, with 10 to call and go to the turn. On the turn you assume he will bet 2/3 pot, or 23 into a now 35 pot.
Implied odds are: 25 + 10 + 23 to 10, or about 5.8 to 1 on your flop call