Fiatscrew,
Thanks for putting in the max 12 ga pressure as it reminded me that not all shotguns were proofed at the same pressures. Keeping it short and simple, my Beretta 12 ga factory proof pressure was 17,000 + psi.
This was higher than some other 12 ga proof pressures.
Every little bit counts.

I concur with the comments which recommend playing it safe.

Also, a nice benefit of the .45-90 is that it can be had just by extending the chambers of a 45-70.
Also, you can still shoot 45-70 factory ammo for little game such as deer and Pronghorn.
Then use the .45-90 for bigger b
game as needed. Or just load in the 45-90 case to keep pressure down.

That was my route with My Beretta 45-90 and it still shoots .45-70 fine.

Another recommendation to keep pressures down is to use VV N133 which usually generates 12,000 PSI less pressure than other powders at the same velocity !!! I bought Pressure Trace II just to check peak pressures in some of my rifles.

+ slow and easy will provide the best solution for your rifle.