Your problem is not unusual withergyld. Most of the 2a No1MkIII Ishapore Enfields I have encountered have headspace issues which lean towards excessive. Both of mine will just barely close on a SAAMI spec .308win "No-Go" guage. But this does not concern me to much, for the underlying reason you are concerned...
The rifle is a 7.62x51mmNATO spec chamber, which has more slop than .308win.
The Enfield actions from square one are loose in design. They are notorious for having headspace issues as well as being rough on brass. Sure someone out there in forum land will say "Mine is perfect", but for the most part it is a common issue on Enfields. We judge whether headspace is safe typically from SAAMI measurement specifications. If its a 7.62x51mm Rifle... it is still yet a different set of specs. More tolerance than a .308. To complicate this further, the Enfield actions as a whole will be governed by their own "Wartime acceptance" specs. These (in .303) were much wider than SAAMI.
The base No1MkIII bolt heads were manufactured with a relative loose tolerance, erring towards long. To set headspace, the face of the bolt head was sanded/filed/stoned by the armorer to fit the rifle.
On the No.4 variants of Enfields, there are a series of different bolt head sizes. Each numberd from "0" to "3". Starting at 0 as base size, each number is a thicker bolt head by 0.003" with some mild manufacturing tolerance which may add +/- 0.0005 to 0.001". Because the headspacing a No1MkIII was a PITA... they came up with this.
My suggestion is to take your existing bolt head and measure it with a dial/digital caliper. Specifically the distance between the bolt face to where the threaded stem starts. This is going to be the determining headspace distance.
From the math you had listed, it seems like you need a bolt head which is 0.006-0.008" longer.
I would then write or call http://www.ssporters.com
, and explain your problem. Tell them what you need. They may be able to send out a bolt head close to the tolerance you need. They charge $10 for the bolt head on their website.
I have had great results with this company, their prices are good and their staff is knowledgable. I have used them on a variety of Enfield restoration projects. I plan on using them again once I move into doing 7.62x54r Chamber conversions on No4MkI ruifles.
Hope this helps,