Well, I crunched the chamber pressure numbers & they are as follows:

Baseline donor gun 12 ga. 2&3/4 chamber: .8100/2=.405 head/base radius
3.14*(.405*.405)=.5150385 square inches of area
.5150385*11,500psi SAAMI spec= 5,922.9 lbs. applied to breech

45-70 calc.: .505/2=.2525
3.14*(.2525*.2525)=.200194625
.200194625*28,700 SAAMI mean value=5,745.5 lbs. applied to breech

.35 Remington: .458/2=.229
3.14*(.229*.229)=.16466474
.16466474*35,600psi max SAAMI spec= 5,862.0 lbs. applied to breech

The .35 Rem. gets honorable mention, because the 45-70 values I referenced aren't even the highest- test loads available for the 45-70, those going over the baseline value of 5,922.9 lbs.

I think the relationship here between head/base diameter and pounds of force applied to the breech relates to surface area. A 12 bore spreads the force out over a larger area vs. centerfire rifle cartridges that are very intense, in a smaller package/with a smaller footprint.

This kinda' bums me out...I was hoping for a larger caliber. But something like the .416 Rigby I calculated at 14,161.3 lbs. applied to the breech...way over.
Are there flaws in the calculation?...Well, if you sub in even 17,000 psi in the donor gun calculation for the SAAMI spec, it produces only a mere 8,755.6 lbs. of force applied to the breech...
It's because the force is spread out over a larger area in square inches of surface area of the cartridge head/base diameter.
That's my thoughts. Thanks for listening.