If pressures are a concern, you can send shells to Ballistic Research (not to be confused with Ballistic Products), and you will get a pressure and speed report back. Load data based on math or cyber experts can be dangerous. I got a load from a reload guru that was of  by 5000psi over max for a 10a. 
 
Ballistic Research phone number is: 815-385-0037. Ask for Tom.
------------------------------------------------------------------------------------------
Tom Roster's services are invaluable for questionable loads
i stick with reliable published sources -  you are correct - too many "experts on the web"
for low pressure short ten loads -  one of the best sources is the series of "Finding Out For Myself" articles by Sherman Bell published in Double Gun Journal a few years ago.  He pressure tested the loads. (he also tried to blow up on of those dangerous damascus barrels - a rusty set of Parker tubes,  it took over four time the SAMI load limit to do it.)
for the original posters question about 12 gauge bismuth loads, I doubt you can do better than the BPI or Precision's book
the old guns are a lot of fun,  and truth be told - many of them have probably digested loads over the years that would make a knowledgeable shooter duck for cover and have held together.  I know the chamber length and minimum wall thickness on my guns -  
I was on a clay course at a classic double gun event once when the shooter ahead of me touched off a load that was very loud and set him back a few paces.  turned out he somehow got a 3 inch steel load mixed in and fired it in a 2 1/2 inch double.  He had the gun checked out and there was no damage.  Darn lucky and not recommended, the next one could send the ribs flying,  
on the other hand - I hunted for many years with a 1904 Parker that was rebarreled in the 30's by Parker/Remington with modern steel 2 3/4 chambers,  after shooting modern standard loads in it for about 10 years I noticed the stock was cracking at the wrist.  once the new stock is on- it will be good to go for another generation.
so-  whats my point?  -  err on the side of caution for the guns sake if not for yours,  load speed and shot size kills birds, not chamber pressure