[Dire]

Yeah, my post was too wordy.

1. Estimate how often he bluffs the turn. Call that x.
2. Estimate how often he fires a second barrel on the river overcard. Call that y.

x*y is his river bluff percentage. If it's greater than .33, you must call.

Complication: This does not account for the times he was bluffing with a queen or flush that hit on the river. Could compensate by adding a few % to your required % for a river call. Regardless, if you really do think he is bluffing this turn rather frequently, you will usually end up with the equity for a river call.