Headspace is a term that has been around since the earliest days of cartridge firearms. It refers, literally, to the space in the firearm where the case head goes.
In the beginning, all cases were rimmed and the first ones were rimfire. The case "head" was the rim, and headspace referred to the space the rim fit in, between the closed boltface and the barrel.
When speaking of rimmed cartridges, which headspace on the rim, where and how bottleneck, rimless and belted cartridges headspace is not relevant.
Specific to the .303 British, and the SMLE rifles, what matters for headspace is where the rim fits. That's all. For military purposes, the rest of the chamber simply has to be big enough for the round to fit into, and being oversized, a little, or a lot, makes no difference at all. An SMLE could have headspace in perfect tolerance and have a chamber grossly oversized and its still fine for service use.
And this is what leads to short case life in SMLEs. The sloppy fit of the case in the chamber, combined with the "min spec" dimensions of sizing dies, and including the fact that ammo is made to be just under "min spec" in size (to ensure fit in min spec chambers.
Add these together and the cases get stretched alot, sized small, and stretched again with each firing/reloading cycle, so they just don't last long, compared to other rifles with "tighter" tolerances.
There are a few commercial .303 British rifles, and their chambers are cut to the usual commercial tolerances, and there are no reports of excessively short case life reloading for those rifles. The problem is that the SMLE is simply NOT a reloader friendly rifle.
You cannot solve this "problem" short of rebarreling the rifle, but you can minimize the effect with careful reloading techniques. And. I wouldn't bother with any sort of "comparator" measuring device(s). Simply neck size fired brass only enough to properly hold a new bullet.
A little trial and error is all that is needed, using your rifle as the "gauge".
Case life won't be great, but done right, you should be able to get 5 or 6 loadings vs. 2-3.
Another point, don't trust "once fired" .303 brass to have its full possible case life. Used to be, I would get "once fired" .303, FL size it once, then neck size only, after that. Then I had a batch that were "fired once" and I FL sized them, and got full case head separation on the next firing.
Now I use only new brass or "once fired" brass from factory ammo that I fired. I don't shoot .303 much anymore, but back when I was doing it regularly, testing new or once fired cases in the rifle to ensure chambering, then sizing only enough to take a new bullet and staying within "normal" pressure limits for that round got me 5-6 loading/firing cycles from the brass, USUALLY. Sometimes a bit more, sometimes a bit less.
Forget all the rest of the info about headspace and how to set your dies using extra tools, gauges and benchrest precision techniques, FOR NOW...
Try the simple stuff, first, and if its doesn't get you anything better, then look at more ...refined...techniques.
Good Luck.
Hope this helps.