I hate to say “stewm*c”, but they have nut seating files.
I bought one way back when (when £1=$2).

https://www.stewmac.com/luthier-tools-and-supplies/tools-by-job/tools-for-nuts-and-saddles/nut-seating-files.html
Use that carefully and the bottom of the nut slot should be flat, and perpendicular to the end of the fretboard.