I posted this reply on the H&H topic. I guess the topic has moved on but in referance to the original topic posting:
It is perhaps interesting that I have never come across any one in the English trade who took much notice of the WT at 9" other than barrel makers themselves. Perhaps I mix with the wrong people (!) but everybody seems to concern themselves with MWT throughout the barrel length, I certainly only quote this measurement on my stock descriptions. This is probably not very scientific but has served us well till now.
I would agree that in a perfect world, a MWT of 0.023-0.025 is the ideal but then I have seen a 1970's H&H Royal, not a light weight model, with original MWT of 0.019".
In my experience, other than in muzzle loaders or very early breech loaders, the thinnest point of the barrel is usually a little more than 1/2 way down the tube. If you work on a MWT of 0.025" at 9" from the muzzle, it might be in the upper teens at 15".
I have successfully nitro proofed Damascus down to 0.014" which, although not to be recommended, was an enlightening achievement.
