You want to compare what you can win with what you're risking.
Therefore just compare what's in the pot to what you need to call.
If there's $3 in the pot and it'll cost you $1 to call, you're getting odds of 3:1 so you need equity of 1/(3+1) = 1/4 = 25%
If there's $4 in the pot and it'll cost you $1 to call, you're getting odds of 4:1 so you need equity of 1/(4+1) = 1/5 = 20%
etc
The amount you call can't be counted as winnings.
The Mathenoobics of Poker - Pot Odds