Trail Boss is a good powder for light loads, but some people have had trouble getting it to meter consistently. If you use a mechanical powder measure, check you charges with a scale regularly, and inspect each round before seating a bullet.
If you have poor accuracy at first with TB using starting loads, chances are it will get better as you get closer to maximum charges. True for most powders, but especially so for TB.
Trail Boss should never be compressed, not an issue in .45-70, but make sure your bullet is not compressing it in .44 spl.
Load 'em up and enjoy.