How I set my timing is with no vacuum advance and rev it up until the mech. advance is all out-like 2000-3500 rpm's depending on how strong those little springs are in the dist. Then turn the dist to give more timing until the motor starts picking up no more speed or running a bit rough or slows down. Then back it off a bit and lock it down.
Afterwards I looked with a timing light and its like around 38 degrees, give or take a little. Time the motor to what it wants, the most timing while still running smooth