I looked back through my references and found the 1982 edition of
The Gun Digest Black Powder Loading Manual, by Sam Fadala. This has one page for each of the three calibers in which the Jonathan Browning Mountain Rifle was produced. Here is an amateur photo of the page concerning the .45:
View attachment 140637
...and a closer look at the table, just for clarity:
View attachment 140639
Mr. Fadala evidently tried only one ball size (.445") and one type and thickness of patch (.015" Ox-Yoke Blue Stripe, by which I think he means ticking). However, I found it interesting that the table shows both volume equivalent and actual grain weight of his various powder charges. There may be more information about the rifle and the test parameters and results elsewhere in the book, but I don't see much about accuracy on this page; only that the relatively mild 50 grain load was most accurate, yet he suggested the 90 grain load would be best for hunting.
Best regards,
Notchy Bob