I've only been breech seating bullets for a few years, so not an expert. I normally start 1/16" ahead of the chamber, or case mouth.
Powder charges usually need to be increased over fixed cartridges due to the extra case capacity that you get seating the bullet this far out. But I started with fixed data and gradually increased it until I got the results I wanted.
You can breech seat with a inexpensive push seater like the old Ideal styles, or buy a mechanical seater that gives you a leverage advantage, and makes seating much easier, and more accurate too. Ballard rifles create some issues as far as which seaters work well, and often back in the late 1800's gunsmiths attached a pin to the side of the receivers to hook a breech seater to, and that's not something I care to add.
I've got 4 different styles of tools I use, and some work excellent, while a couple work OK, but get tiresome after a short time.
A stepped bullet, or tapered bullet, makes breech seating much easier, and they shoot really well also.
This is my Cleave breech seater:
(You need to Login or Register to view media files and links) This one is either a Glen Goerzen, or Bud Barnes tool:
(You need to Login or Register to view media files and links) This is one I make for myself:
(You need to Login or Register to view media files and links) This is an Ideal push seater at the bottom in this pic:
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