Yer carnarf go orf some people!
I could really get to hate Roger Moss ‘coz he can type faster than me and wrote in his first posting on this topic almost exactly what I was about to submit… Grrr!
Steel rulers can & will squirm about like a worm that has seen the hook and unless the engine is on the bench the flywheel is not that easy to get at when in the bike.
So rather than measuring around the diameter might I suggest using a pair of dividers and a bit of trigonometry? To get the chordal distance for any desired advance angle we need to briefly go back to school.
Find the Sine of half the desired angle and multiply it by the radius of the flywheel (4½in.) and then double it. Carefully set the dividers to this figure. Place one point on your TDC mark and scribe a mark on the periphery with the other.
My inclination would be to use an accurately located centre punch mark for TDC as the dimple will provide a positive location for the divider point to pivot in. Once the advance position is found another ‘pop’ mark there will provide a more lasting advance mark. For the really technical that have access to a strobe light a drop of paint on the rim with the line scribed through it will assist checks with the engine running.
Worried about trig functions? Microsoft Windows provides a calculator, that will do them and most hand held calculators do too or if you never throw anything away there are always those tables you had at school.