Example 1: Half-pot bet
The pot is $100 and your opponent bets $50. You call $50 to win a final pot of $200, so you need 25% equity. A flush draw on the turn has about a 19.1% chance to complete on the river, so immediate pot odds alone don't justify the call.
Example 2: Quarter-pot bet
The pot is $100 and your opponent bets $25. Calling $25 makes the final pot $150, so you need 16.7% equity. A clean nine-out flush draw has about 19.1% equity with one card to come, which clears the break-even point.
For an exact hand-vs-hand result, use the free equity calculator to find your actual equity and compare it with the required equity above.
Use the free poker equity calculator