I'd think you'd dial 3.0 mils the first time, go -0.1 to get to 2.9, then go positive 0.1 to be at 3.1. But yes, at some point there would be a threshold where you'd have to dial the fine turret back, go up a mill, then come back down or whatever.
unless a click of the 1 mil turret always resets the 1/20th turret.
if it works like i think it might, its a useful idea for some applications.
or just buy a night force and be done with it.