The belted magnums are really simple. You set the belt tight to .002 loose. Then neck size or bump the shoulder back .001 or .002. The shoulders vary on reamers by as much as .010. The first firing can be hell on a magnum and the often suffer head seperation. Most guys just keep the shoulder tight after setting the belt(headspace) tight.

http://www.larrywillis.com/headspace.html