The min thickness spec is for machining. If they are smooth at 20.8 then no need for turning, just run 'em. If you want to get them turned, then fine. Get them turned as little as possible, the shop will go as far down as the min thickness and no more. Use them until they get grooved, etc. It should be no more than about $10-12 per rotor, if you bring them in to a machine shop.