So I am struggling a bit with defending postflop against Cbets.

On the river if our opponent bets full pot in a balanced way he should be bluffing 1/3 of the time to make our call with a bluffcatcher indifferent, right? Because we are getting 2-1 we need to win 33% of the time. If he bluffs 1/3 of the time we will be winning exactly 33%.

So how does this relate to the flop? Lets say the pot on the flop is 12$ and our opponent bets 8.50$. Now on the river we would be defending about 29% of the time (8.50$/(12$+8.50$+8.50$). But to me it seems kind of strange to defend less on the flop against a smaller bet instead of more. I understand that hands on the flop still have equity while on the river they either win or lose. But what kind of impact does this have on our flop play?

So how do we decide how much we should be defending on flop (and turn)?