I had an old H&R Huntsman in .58 and could never find a factory recommended max load for it. Lyman's BP handbook quotes H&R as recommending 100 grains FF as a max for a 505 grain minie, but that's it... nothing on any other projectile. I worked up to 140 FF with a patched round ball and it never came apart... Elsewhere in the handbook, Lyman specifies up to 180 FF with a patched round ball in their Universal receiver.
Maybe we can just work at this empirically, and compile a list of "recommended maximums" & keep staring at it until some pattern emerges? [Gotta love the human brain, the integrating engine...]
So lessee, you've got 100gr BP behind a 505gr bobo, and I just came across a statement from an old manual that came w/ a BP pistol to the effect that "It has also been proved that maximum loadspeed perf. for a caliber .45-.50 with an approximate 28" barrel length is about 70gr of FFFg powder."  I think it is probably safe to assume that this is for firing a round ball [nowhere in the manual are conicals or Minies mentioned], so let's go w/ an approximation of 175gr for the bobo in this example.
In addition to this example, I've heard that the old approach to figuring your load was to hold the projectile in the palm of your hand and pour powder over it until the bobo was juuuust covered/hidden. This of course assumes that BP has a consistent density, which might be not absolutely
true, but thanks to BP's general forgiveness in this realm, seems to work.
So to recap, thus far we have:
Anyone else care to contribute?
To give you an idea of the age of the manual, they list 5Fg powder as being the stuff that passes thru the 60 mesh that is the bottom
"do not pass" level for 4F powder. Hmmm, seems like times have changed! ;-)