Guys- something that might help in the loading and pressure business, is to obtain Hodgdon's Annual Manual. Yes- it comes out every year. I bought the last one directly from Hodgdon by a simple phone call. As well, Richard Lee's handloading book might be of use.
Note Lee uses the same "any .45/70" data as his 11mm/.43 Mauser data as well, however the .43 Mauser has a 77gr. capacity and used 340gr. bullets. I have used this data in an original M71 Mauser (1876) that I rechambered to .450 Alaskan (97gr. case capacity). I chambered the rifle to .450 Alaskan so I could get good cases at the time. .43 Mauser brass was not available, however it is now- apparently, from Star Line. I load black powder level speeds using smokeless powder, my choice being H4895 with foam backer rod for filler. With the loads I am using, I get almost identical velocities as shown for the .45/70 - even though my case is MUCH larger. Same velocities means similar or same pressures, certainly nothing dangerous.
It is best to also use a chronograph with your load development.
In either of these books, you will have to extrapolate, but - good starting loads for .45/60 and .45/75 can be found in the Trapdoor Springfield data (HODGDON) using their starting loads as starting loads. The Hodgdon manuals are super as they list Hodgdon, IMR and WW powders- including Trail Boss in many ctgs. even the magnum modern rounds.
Note the pressures that can and are generated with Trail Boss. The might seem like go-pow pistol-type loads but many of them are in the 24,000PSI range, especially when dealing in the 100% loading density arena.
------------------------ DO NOT START WITH TOP END LOADS SHOWN FOR THE TRAPDOOR. ABSOLUTELY DO NOT USE DATA FOR -------------------------THE STRONGER ACTIONS IN REPRO ACTIONS OF LESS STRENGTH. The only rifle I can think of as possibly fitting the stronger bunch such as the Marlins, are the M92's and M86's. The MFG're should have the final word on that.
Note the pressures in the 15,000CUp to 16,000CUP range - THAT is where to start. For many of you, those velocities are about where you want to start and likely stop to obtain original black powder load levels. Yes - those smokeless loads develop LESS pressure than black powder loads do or did in the same cases, with the same bullets.
This data is very useful. Note that it takes a BIG change in capacity to make a difference in pressure and velocity delivered.
You cannot go wrong with what I have stated above.
Also- some might want to try jacketed bullets with black powder. Don't laugh. It works. My bro shot a moose with a 500gr. Hornady RN with his Sharps .45 3 1/4, loaded with 100gr. 2F and a whack of 1/10" wads to fill the space. It worked. In Australia, there is a rifle competition which allows jacketed bullets with black powder loads. Try it if you want - much better accuracy than with cast bullets and black powder, usually. Surprisingly enough, there is no fouling buildup as the jacketed bullet just pushes it out, each shot. Now, if you have a loose spot, it will build, I suspect, but do not know.