Thanks everybody. Further suggestions welcome. Indeed, I do not have $200 to spend on a gauged file set, however I have a cheap set of needle files, one of which is a tapered round file. I had to redo the slots on the ivory nut I recently had made for my Strat, and the round needle file worked very well. Believe it or not, after some minor reshaping and as I said redoing the slots, I believe I was able to get to within thousands of the target slot diameter. Also, I was able to cut the slots not only round, but so that the strings seated only half way in the nut slot. This is my preference, and I believe it agrees with nut slot lore.
Guit Boxes: 87 MIJ Strat, Ibanez MIJ RG540, Korean Fender Dreadnaught, The Loar LH-500 (1934 L-5 Reproduction)
Amp: Marshall TSL100 amp head with JCM900 1960 Lead 4 X 12 angled cab
Effects: Crybaby, TS10 Tube Screamer, Badder Bad Monkey, Boss Metal Zone