[EricH]

Bearings need a pretty tight tolerance because they need to be pushed in. 0.75" is as small as you can go with the hole, so you really want it slightly bigger. I don't think there is an equation, at least, I can't remember one.

I would assume that the shop can get to within 0.005" fairly easily in metal. That should be enough--if the hole for the bearing is 0.755" in diameter, I would expect the bearing to press in a bit, but stay pretty well. Smaller is better, though, because it's pretty easy to remove material that is there with, say, a reamer, but pretty hard to add it back. If they go down to 0.001" tolerance, that's even better.