Chip:
I agree with Mike. A bit big is not a problem, especially once they are pinned and lashed. Besides, a little extra room is better than none, especially when fitting steam-swelled ribs into the mortices.

After two SOFs, I broke down and bought a morticing machine (it was actually a good deal, $99 through Harbor Freight). It is really nice to be able to cut perfectly square holes to a uniform depth quickly. Much nicer and more accurate than the drilling jig I used (similar to yours).

Good luck,

Marcel