Take the cap off the plug...Turn the ignition on..slowly rotate the crank CCW..When you hear a spark draw a line across the hub and case...Put the piston on TDC...The distance from mark to mark should be hub diameter x .244....the spark happens when the edge of the magnet passes the edge of the sensor...The width of the sensor and possition in the holder can vary, so you can't tell exactly where it fires if you try to use the other measurement method...Using the spark to do the timing is better...You won't hurt most ignitions turning it a few times with no spark plug..If you're worried about it put a spark plug in the cap...
Timing is not exact, a few degrees one way or the other makes NO difference at all..
I have only been doing this for 20 years, maybe all those engines are timed wrong ?
The timing on the Chinese ignition works out to about 28 either way....