Also, for simplicity’s sake, let’s assume that each time we make the flush on the river we win the pot (we would even have the best hand ~96% of the time). This means, when you hit the flush on the river with K♥️ Q♥️, you'll need to win more than $25 to make the turn call worthwhile. Since the pot is already $50, you'll solely have to extract a half pot guess on the river on common.
If you bet $20 into a $20 pot as a bluff on the river, pot odds dictate your bluff have to be successful 50% of the time to revenue as you are risking $20 to win $20 (1-to-1). Instead of getting to win the pot 20% of the time, the pot is now laying us 2.5-to-1 odds, aka 29%. Given that our chances of improving are round 25% we wouldn't have direct odds to call. You are heads up on the river with $45 in the pot and your opponent bets $5. Written as a ratio this would be 45-to-5, simplified as 9-to-1 or 10%.