The bottom of the nut slot should not be a semicircle the exact same diameter as the string. That will cause unnecessary friction when tuning. You really only need contact at the bottom of the string, and a slot that isn't flat on bottom so the string won't slop from side to side. The bottom of the slot can be a semicircle (or some other concave curve) quite a bit wider than the string and do that.
A knife edge needle file can do the plain strings from .010" to .017". As others have said, it's easy to add a few thousandths by tilting the file a bit from side to side as you cut.
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