Basically it is difficult to "blow up" a well made original or replica BP rifle if you use just a little common sense.
And then you can always use the Dixie Gun Works "proof Test."
In a nutshell, you research to see what the "typical" load you might use (you are well on your way, somewhere between 50 and 70 grains of FFFG is about right for a .45), then DOUBLE it, ram TWO patched balls down tightly on top of the charge, then bungee cord the rifle to an old tire, with the buttstock inside the walls of the tire against the inside of the tread, tie a 20 foot string to the trigger, and pull it from 20 feet away...it goes boom and it stays together you will then be able to shoot it knowing it will stand up to ANY "normal" load you throw at it.
The caveat is almost ALL original or repro guns will survive this test with no problem, and if yours blows up you aren't out anything....the piece of "pot metal" could have blown up with you AROUND it plus you then have REMOVED it from the "gene pool" LOL
The other test that works well to find the MAXIMUM load your rifle can use is to shoot it with progressively larger loads of powder over freshly fallen snow (assuming you are in a climate that GIVES you snow
)
You will find the patch laying a few yards in front of you, which will tell you whether the ball is too tight, the patch is too thick, etc, but the main thing is when you find "pepper" (It IS very noticeable) in front of you on the snow, you have reached the maximum useable load of powder in your gun for that ball....the "pepper" is unburned black powder that got pushed out of the bore before it burned. Using any more powder than a few grains less than when you started seeing the "pepper", is wasted.
For example, I hunt with a flintlock Numrich Hopkins Allen Minuteman "Brush" rifle with a 24" barrel....my load is 60 grains FFFg behind a .440 ball with a .010 thick patch...that is the most accurate of the heaviest loads, I can go to 65 grains, but at 70 I start seeing "pepper" over snow. With your 28" barrel I would guess you could maybe hit 70-75 grains, but most BP rifles shoot more accurately with LESSER charges...so 50 just might be a good load for yours, and it will handle it forever...
The "Mountain Men" didn't measure BP all that much at all, and generally used a LOT smaller charge than we generally do to save powder....what they did was put a ball in their hand, and carefully pour powder OVER the ball until the ball was covered..which general worked out to be about one grain per caliber...so for a .45 ball a typical period load from the early 1800s would be somewhere around 45 grains of FFFg....