Lots of research went into this and finally I have found the way to do this.
First SAAMI specs are too tight from and always have been.
This has been stated by numerous Enfield experts and differ from the arsenal specs given by the manufacturers of Enfields.
That being said the proper headspace for an Enfield as per Commonwealth specs is as follows:
0.064 to 0.074 with 0.084 allowed for a War Spec.
Enfields are built to allow the manufacturers of ammo some leeway on the case rim thickness.
Since most American Manufacturers build to the SAAMI spec most will not pass European inspection as the rim is too thin.
Why allow a thicker rim?
Breach machining tolerances.
A perfect depth breach cannot be machined and will vary slightly. Even today’s machines cannot produce a depth that is exactly the same all through the circumference of the bore.
That is why there is run-out, and depth specs allowed for any round area or bore.
So back to the Rifle and the 303 Brit.
How do you measure it?
This is best explained with a drawing.
I did this in excel and by no means to scale.
First remove the Extractor.
Second take a fired cartridge and insert it into the chamber.
Next use a piece of solder and lay it across the cartridge face where the bolt will interface.
Leave a length to hold on too.
Close the bolt on the cartridge and then open.
The solder will smash to the size of the gap between the bolt face and cartridge rim.
Measure the rim in several places of the cartridge and average it.
The two added together is your headspace.
So on my #5 Enfield the gap measured 0.0225.
The Rim of the cartridge averaged 0.059.
0.0225 + 0.059 = 0.0815
So my Enfield is 0.0025 within War Spec and still can be shot safely.
However I want to mention that when rimmed brass is shot it stretches to seal in the chamber.
If I reloaded 303 (I don’t) then full length case re-sizing would eventually keep this brass stretching and getting thinner to the point of failure.
So if your Enfield is like mine then don’t resize the case just the neck.
Or don’t reload it at all as I do.
I have shot hundreds of rounds through this from Cordite charged to Flake powder charged.
I would like to thank all the resources I used to gather this info but there was just sooo many.