if you set the timing on the front cylinder compression stroke while AT FULL ADVANCE & then lock it so it will not advance too much all you do is twist the timer clockwise to retard it, the advance /retard unit adds or retracts the timing 35*(degrees) i beleive it is,
but retard is not as critical as full advance,
if you dont lock the advance stop & ride over full advance, ya likely to do damage in the long term,
plus loss of power & bad MPG,
retard it by hand till you get the idle beat you like, it doesnt really matter as the motor isn't under load,
but IF you follow the FACTORY MANUAL it's so easy, DONT over think it, just set as the book says,
1)front cylinder on compression stroke,
2) get the timing mark on the flywheel in the timing hole.
3)narrow lobe on points cam JUST ABOUT TO OPEN points at FULL ADVANCE,
3) tighten it down & ride
it aint rocket science, it's 100+ year old technology