Re: headspace gauge
The headspace measurement in most(all) of the cartridges I use is from the bolt face (base of the case) to a datum line, usually at the midpoint of the shoulder angle on rimless cartridges. On magnums, unless you have had the chamber custom cut so that it headspaces like a rimless case, it headspaces on the belt, which is really hard to control w/ normal dies.
Most of the factory dies I've had say to screw them in to where they just contact the shell holder w/ the ram at the top of its stroke. That should make the case chamber in just about any gun in that caliber out there. In reality, I don't care about my cases chambering in any gun but mine, so that is un -neccessary overworking of the brass.
As an example of which I happen to have the numbers in my head, I recently screwed a takeoff .243 Win barrel on my Rem 700VS. The .308 Win family of cartridges (.243 Win, .260 Rem, 7mm-08 Win, .308 Win) headspace on the datum line, which measures 1.630" from the bolt face. Fire formed a couple cases w/ light loads, and came up w/ a fired headspace dimension of about 1.635". Screwing in the dies all the way per the directions sized the cases down to 1.619". Waaaaay oversized. Adjusting the die to where I had just a smidge over 'nominal' spec headspace, or about 1.632" still allows the case to chamber fine in my gun, but w/ minimal working of the brass.
You can either use the RCBS Precision Mic tool, which you have to get one for each cartridge you load for (in reality, I never use the bullet seating half of the tool, as I have a Stoney Point OAL Comparator, so I could use the headspace portion for cartridges w/ similar headspace dimmensions like the .243 and the .308). It doesn't give you a number like 1.630", rather you see it as being over or under spec as viewed on a micrometer scale.
I eventually opted to get the set of headspace comparators from Stoney Point, where you look up which cartridges use which insert (based on shoulder angle, I think), mount the insert, zero the calipers (I'm using a set of digital Mitutuoyos), and check the head space.
Lastly, there is the Redding dial indicator die, can't remember the name. Haven't used it, and probably won't w/ the Stoney Point inserts. They are accurate enough for my uses, and at this point, there's no sense in spending $30-40 per cartridge for a Precision Mic or Redding dial indicator.