I know, I have to use a magnifying glass.
Cutting the nut slot, the nut is 16 mm back from the edge of the fretboard, the router bit is 59 mm from the edge of the router, so I set my level as a guide 75 mm back from the edge of the fretboard. Squared it up and ran the router along the level straight edge. I put scraps of MDF on either side of the fretboard and leveled them to the wood with bits of paper. This means the router dousn't wobble as it passes the edge of the wood.
that's not bad, it will need widened up a bit to get the nut in.
Checking, it's square, yep, close enough