you must use a degree wheel to time it,or use the in the hole method.
i made a piston stop with 3/4 square stock,that has the cheepest 5/16 bolt in the center that acts like a piston stop.this is bolted to the head surface,with a head gasket it makes the piston 27 degrees into the hole. without the gasket it sits at 32 degrees.
have the magneto on the engine and set close to flywheel,about .035 away. align the left side of the flywheel magnet to center of the pickup on the magneto,which is about the center of the right side leg of magneto to right side of the flywheel magnet.with my piston stop i can either set a degree wheel up,or just use the 27 degrees of the pisto stop.with gas use the 27 degrees to start.
hope this helps and is not confusing.