I'll put my neck on the block.......
It isnt the DT thats important, but the amount of steel left in the barrel around the bore. We all know that the greatest pressures are in the breech, and dissipate quite rapidly as the projectile moves downbore. If we look at some of the high velo/high pressure bottleneck cartridges on light contour 'mountain' hunting rifles we see the chamber swell, and a rapid radius to barrel, then a long taper to muzzle. I think we could compare the breech pressures and barrel dia. on those and likely see that on the W&H, even with a nominal (often) DT depth of about .090" there is plenty of steel left for any sane load in the rifle. Remember, the DT bottom is flat and the bore circular, so there is only a 3/8" length of the thinnest barrel wall above the bore, ever increasing in wall downward each side.
For me, I'd do the calc. of steel left at that DT and spend a little time online looking for some reference on pressure drop per inch from ignition with a 45-70 (different loads and powders will vary) and see if there is a chart somewhere on barrel profiling (I know Ive seen such in a gunsmithing book in the past) to see how much steel there 'should' be at those pressures. All just to 'know', and probably just a lot of spinnin' my wheels. I do know that a bunch of muzzleloaders have been running for years, some with pretty big charges of black powder, with a powder drum screwed into the actual beech (the high pressure end) with only a few threads to hold. Think about this: a 7/8" barrel in 45 cal., or .875 - .450, /2 = a barrel wall back there of only .2125. A 15/16" barrel is only slightly better in 45cal. at .245 but in the more common 50cal it is down to .218". I know a lot of people load those heavier than a 45-70.
Food for thought, as always, you are on your own.....
