By no means am I an expert on the matter, I have never loaded fora belted mag.I have owned a few, but always bought factory ammo for them. But this is what is says in my Speer reloading manual.

300 Win Mag
Handloading for the 300 Winchester is straight-forward. Like other belted cartridges, the shoulder-not the belt-should be used for headspace control.

338 Win Mag
Like other belted magnums, we recommend that you set your loading dies so the case headspaces on the shoulder, not the belt.

240 Weatherby Mag
The case has the same rim diameter as the 30-06 so rifles can be bulit on standard actions. The belt is strictly cosmetic. Best accuracy and case life is obtained when the case headspaces on the shoulder.