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.