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you'd have to be 66% certain he never follows through with a bluff on the river to justify a fold.

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it doesnt work how you describe here. i'd word it, he has to be bluffing 33% of the time. i dont know the answer to that question, but id guessits somewhere between 5% - 50%, so i folded.

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The % is an aggregate. You're not asking yourself what % of the time you think he's bluffing in this exact spot in this exact hand. That's impossible to determine, you're not a mind reader. You're just asking yourself what % of his entire range in this spot is a bluff.

And this is a conditional probability. By this I mean that his frequency of bluffing on the turn is going to have some direct correlation with his frequency of bluffing on the river.

I mean consider the 5% number you offered on the river. That's just not rational. One possible way to result in a 5% bluff range on the river is to say that he bluffs the turn 25%, and fires again only 20% on the river. Surely you believe that he bluffs the turn WAY more as well as firing again way more than 20%.

You just need to figure out what percents you put him at. If you think he bluffs the turn 80% and fires again 66% then it's a clear call with greater than 52% expected equity in a 2:1 spot! Although this is a naive calculation in that I'm not counting the times he was bluff/semi-bluffing the turn with a Q or flush draw that hit on the river.

In a very long winded way, I'm just saying that the turn and river are directly connected in this spot. If you think he's bluffing the turn, you're basically obligated to call the river since most people willing to bluff the turn in a spot like this are going to be very capable of firing another barrel. When that scary overcard hits, the chances of them firing the next barrel shoot up even higher.