I use a ruler and a very sharp scribe and put as deep a scratch as possible and use this as a guide for the fine files to mark the point.
If you put the edge of the file against the spine you can feel the edge slot into the scribe line,so you know you are in the correct place .
If the cut requires a round file i will start the cut with a triangle shape first to give the round file a larger guide so it dosn`t go where ever it wants I hope this helps
Kind regards peter