The following's a link to a gauge that measures case headspace for the 50 BMG round.
It measures the distance from the case head to the reference point on the case. If you're measuring a fired case that had a normal, safe maximum load in it, you can add 2 thousandths to the reading and that's typically going to be within 1 thousandths of what your rifle's chamber headspace is. Otherwise, to get your rifle's exact headspace dimension, you'll need an adjustable 50 BMG headspace gauge and follow its directions to measure the chamber headspace.
Then use this Giraud Tool gauge to measure your resized fired cases and set the sizing die in the press to make sized cases about 3 to 4 thousandths shorter than what fired ones have.
I've searched the internet on earth as well as Mars and Jupiter and no specific 50 BMG chamber headspace dimension specs could be found. Lots of gauges out there, but nothing as to specs in the traditional manner of measuring them.