No set RIGHT way to know where to set it without a dyno.
As you roll it forward, it RETARDS it.
Depending on compression and fuel, somewhere between 18-28 degrees works good. if he"s high compression, you will have to move the governor gear a tooth to get the mag to work.
Nice thing about a mag is you can set cranking and runnng sparks independantly. You"d have to get someone to change the guts in the distributor to do that.
Never had to have stiffer springs due to compression, just RPM"s.
My mag worked fine at 11+ compression.