Your link does not lead to headspace gauges on my phone, just a sorta generic Midway screen. Probably my phone's fault. For a belted magnum, the headspace gauges are short and stubby, as they space off of the belt, not the shoulder. Set yyour barrel using the go- and no-go gauges. Then you should be good for any factory ammo, as you stated. Should you decide to handload, you would use brass fired in your rifle, and set your sizing die to just barely bump the shoulder on the fired brass. No need to change anything on the gun. After firing, you would essentially be treating your brass as if it now headspaced off of the shoulder instead of the belt. This would minimize case stretching, and somewhat prolong brass life.