Howdy
Yes, Kuhnhausen says that .006 is correct for jacketed bullets and .008 is correct for lubed leaded bullets.
However there is a little bit more to it than that. The correct way to measure barrel/cylinder gap is with the cylinder shoved forward. If there is any endshake (forward and backwards slop) endshake will mask the true b/c gap. That's why you make sure the cylinder is shoved all the way forward before running your shim stock through. Don't allow the shim stock to shove the cylinder backwards taking up the endshake. In theory there should be zero endshake, but that is not always true, a couple of thousandths of endshake is common.
For what it's worth, I have lots and lots of old revolvers. Some of them have gaps much larger than .008. None of them suffers much from accuracy that I can detect. When I buy an old revolver, I don't bring feeler gauges with me, I just eyeball the gap. I certainly would not sweat a gap of .007 or .008.
However I once owned a Cimarron Cattlman with a canted front sight, and I would not put up with that in a new gun.