Put the barrel behind you, place the receiver in front of you, meaning separate the two, then install the bolt and measure from the front of the receiver ring down to the bolt face with a depth gage or a dial caliper etc, what ever tool you have then (Turn around),place a new unfired, minimum length full length sized store bought round in the chamber and measure case head protrusion from the case head to the mating surface of the barrel.
The Remington 700 HAS A RECOIL LUG, MEASUREMENTS MUST BE TAKEN WITH THE RECOIL LUG INSTALLED. Or the lug thickness can be measured and then subtracted from other measurments etc..when installing the barrel to the receiver the lug must be held om place.
The difference in the two measurements, from the front receiver ring and bolt face and from the protruding case head down the the barrel (to receiver) seating surface determines the length of the chamber from the shoulder of the chamber back to the bolt face.
The bolt face does not have a datum, the bolt face is 'measure to' the shoulder has a datum as in 'measure from' , the datum for the 300 Winchester Magnum is .400 as in a round hole that is 2/5" in diameter. If smiths and reloaders understood the concept they could make their own tools, store bought is OK.
You have head space gage, one a go-gage, the other a no go-gage, the 300 Win Mag head space gages are belt only gages. I want to know the length of the chamber from the shoulder/datum to the bolt facc.
Everyone follows instructions, I remove the belt from a test case to determine the length if the chamber. Those that follow instructions are required to fire form a case to determine the effect the chamber had on the case when fired.