Mr Guffey,a problem with using cartridge cases for gages is the cartridge cases vary,and the cartridge case makes a poor gage.
The proper way to set headspace for a belted magnum is off the headspace gage which will then accomodate a maximum loaded cartridge.
It will not do to short chamber to gain a tight headspace at the shoulder of a belted magnum cartridge,as you will not be able to close the bolt on the belt.
The chambering reamer maker puts those dimensions on the reamer.If you have a good reamer,and you put the belt headspace in the right place,then you will be fine if you do not set back the shoulder with the sizing die.
Mr Guffey,I do agree with you about depth mic'ing from the receiver ring to the bolt face..OP,make a drawing,then you won't get confused.
Receiver ring to bolt face,add recoil lug thickness.
The barrel shoulder butts on the recoil lug.Barrel shoulder to the end of the go gage length will end up equalling recoil lug thickness plus receiver ring to bolt face.
If you want to sneak up on it,do the same measurements with the no go gage.Note the difference in length marked on the gages.
So,you can practice by fitting it to the no-go,and if you hit that just right,then you know how much more to take off.If the no-go won't go,and the go does,well,its acceptable.Maybe hand tight on the no go,a feeler gage between the barrel shoulder and recoil lug will tell you some.
Just for fun background,that belt came around from when the British were loading cordite sticks.The load was so many sticks of cordite
Those were loaded into basic,straight brass.Then the brass was easily necked down with the shallow shoulder angle of the H+H cases,the bullet was seated,all was well...except it made for unreliable ignition as the shallow angle was "soft".The belt was added as a good anvil to strike the primer.
But,a belt is so positive,about .005 of clearance was designed in,at minimum,so when Mbogo was going to stomp you,you could count on chambering the round.Reloading was not a consideration.It had to work just once,every time.