1 turn either direction is about 1" difference in vacuum. I just get the vacuum advance max to 48* (34*+ 14*) Vacuum Adv at highway cruise. You can then adjust when the can starts to move by trial and error. If car pings street driving go CCW until it stops. Advance CW till it pings, then back off.
@Mattax did a thread on distributor tuning but I can't find it.
