never fold the set to a small river bet, ever.

I am not certain, but I think that the math you want to be working out here is how much can you call so that your opp does not win enough (implied odds wise) to justify his earlier calls. Also taking into account his percentage of having a different draw than the one that came in and how often he can bluff you out by repping that draw. That will get you a much more solid number, one that doesn't rely so much on your feel on the river.