i use 20 gns of swiss 3fg in my pietta 1858 revolver with .454 round ball but i use to use 25gns......i weigh mine rather than measure but each to their own.

i use 70 - 75 gns of swiss 3fg in my martini henry pushing a 500 gn cast bullet .
some use 2fg with good results.

with my pietta i can use .457 ball too like your ruger old army but i prefer .454
next time at the range i will use .452 lee conicals with 20 gns of swiss 3fg
only issue is with 777 which is good powder to use but doesnt like compression as it can pressure spike