The headspacing on the belted magnums is determined by, and measured to the front of the belt. This is what your headspace gages will set.
The belt may have been desirable on the .375 and .300 H&H due to their very shallow shoulder angle but they are merely superfluous on any cartridge with a pronounced shoulder.
The belt was however very good for marketing purposes in the 1950-1960's when a whole bunch of belted magnums made their debut.
Set the headspace with your gages initially but after you've fired a cartridge in that chamber you don't need to size to the belt at all. Treat them as normal cartridges and use the shoulder. If you do size all the way to the belt you'll likely be working your brass too much because you'll be pushing the shoulder back every time.
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