Won money when calling at showdown

[Benman]

Poker Tracker says I win money 32.14% of the time when I call a river bet. Something tells me this is quite a bit too high to be optimal, that I should be calling more bets for the pot odds. How can I calculate what the optimal number is? Unfortunately I can't seem to figure out on Poker Tracker what the average pot size is on the river when I call a bet--that would seem to be a useful stat in this question. Any comments appreciated.