Been out of town for awhile...
OK the syncro spark DOES measure dwell to determine RPM on the first turn. The "magic " number is 180 rpm. Below that its fully advanced to allow proper set up, above that it is fully retarded.
It is "calabrated" to a 3/16" magnet on a 2" hub. Useable magnet is 1/8"-1/4". If the magnet is to large the dwell time is increased and the SS thinks your at a low RPM and stays advanced. SSSSoooo it won't work with magneto sized magnets. As the Dia increases the dwell time decreases so you are safe. NOTE the rpm is the same, but dwell decreases... think of a wedge of pie. The number of degrees in the wedge stay constant but each degree covers more distance. so the dwell of the same sized magnet gets smaller the larger the dia.