AJ and the opinions that follow are correct. I have extensive knowledge in this particular topic.
Suffice it to say, whether the cartridge is belted or not, any custom rifle
maker headspaces off of the shoulder, not the belt. Sure when the belted magnums first came out( decades ago) they were headspaced off of the belt. I have no idea what the factories are doing now, my guess is the cheapest thing.
Hope this helps.