For bass nuts, you can normally use a combination of needle files. I have just purchased some Hosco bass nut files, but I was using needle files before that. I find the round one to be pretty good for lowering the slots, just using the tapered end of the file to get the right diameter for the slot - though you need to work from both sides otherwise the slot is too tapered.

Don't cut the blank so it's over-high, otherwise you'll be filing for hours and hours.

I have a cheap eBay string-spacing ruler. It's pretty much the same one as StuMac sell, but less money.