Jordan, The gauge is fairly important here. Belted magnums need to be fairly close at the belt, like you stated. If you don't have a gauge and you want to set your headspace anyway... Go ahead but make the belt close. Usually belted magnum shooters know the first time the brass is fired it will grow between 5 and 10 thousandths at the shoulder. From there, neck sizing is good untill the bolt will no longer close. Using a case comparitor of some sort is a valuable tool to learn about your chamber.