The very best advice I received from almost every experienced BPCR shooter here and on the Sharps forums was, as a beginner in BPCR, to go with the .45-70.
The only exception to that advice was from Don McDowell (Ranch13 here on the 'Fire) and he is big into the smaller cal BPCR cartridges. He's just prejudiced.
There are volumes of data and ballistic info on the .45-70, it only uses a half of a boxcar of BP a month (versus a full boxcar for the .45-110)
, and will get you out to 1,000yds. It is easier to get an accurate load with than the big .45's as they aren't quite as temperamental about load column.
I'm not one to offer load column advice as I am still a beginner on this BPCR journey, having only been doing it for a few years now. Woody (FlyboyFlem) and 45_110 here on the 'Fire are a couple of the local experts. For some great advice, ping sharpsguy, Sharpsman, Ranch13, Paul39, Kurt71, Otter, sharps4590, saddlering, EvilTwin, and/or pacecars, just to name a few. Those guys have been doing it far longer than most of us.
In fact, if my memory serves me right, sharpsguy and Sharpsman invented black powder.
Ed