If you are turning the bolt down on the rocker to get 21lbs with the stock ground cam and IF the aftermarket cam has a smaller diameter then the OEM cam, but you still tighten the nut until you get your 21lbs, the bolt you are tightening is accounting for the smaller base cam diameter. It may take another turn or two on the bolt to get the pressure you are seeking, but is the 21lbs you putting on the bolt that is setting the preload on the lifter, not the number of turns of the bolt. As long as the rocker isn't binding or making contact with the head when you get to the desired pressure, its all good. You are over thinking it.