[geezer#99]

Idle rpm can effect your initial timing readings. If idle gets close to where the mechanical timing starts then your initial number may not be accurate. Mechanical can start at 750 rpm.
Best to get idle as low as possible for initial timing checks.
Seems your vac pot does add 20 degrees.
A limiter is cheap to buy or make one yourself.
I usually adjusted them to give 10-12 degrees.