[Nick C]

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if we expect him to buff more than 30% of the time we should c/c.

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Since this is not possible unless Villain is betting some hands he wouldn't call with, what you're basically implying is that we should check-call if Villain will bet anything he would have called with while also betting a few pure bluffs. And I agree with that, based on your assumptions.

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if we expect villain to bluff less than 12.5% of the time we should c/f.

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If I'm understanding your numbers correctly, your cutoff point is about 6 percentage points too high here.

Edit: And, all right, I guess I should explain why. First of all, the 12.5 number is off; for "optimal" bluffing frequency relative to the pot size, the number should be 1/7 of 70 percent (the 70 percent being the winning hands he's betting), or 10 percent.

Also, though, even if Villain doesn't bet quite as often as is theoretically optimal, he still gains when he bets the worst hand and we fold. So it takes a while for check-folding to overtake bet-folding, given all of the assumptions in the scenario. The reason is, basically, that although we're betting with the worst of it, at least when we bet-fold, we never surrender a pot we should have won to Villain (and this of course assumes he never bluff-raises -- if he does bluff-raise sometimes, then that could change things dramatically). And it takes quite a few saved bets to make up for the occasionally sacrificed pots.