Re: adjusting die for a belted magnum?
"I have the tools from Hornady to measure headspace and COAL." Those will do the same job as the RCBS Precison Mic, use them instead.
Forget how the factories cut a chamber for the correct headspace. Rims and belts are irrelivant to reloaders, just use your Hornady tools and your precision caliper to set your sizer die as it should be, it's all done the same way for ANY bottle neck case.
Measure a few fired cases to find the head-to-shoulder length. Absolute precision isn't required but you do need to establish the relitive number with consistancy. Once you find the longest fired case, simply size so the finished shoulders are no longer than that longest length and you're done.
You will find some variation in individual cases, both fired and sized. That's due to differences in the springback of individual cases. DON'T 'average' them, that only obscures things, just make sure your longest resized cases are no longer than the fired ones.