I myself would be happy with the top velocity you are getting (with a hard cast) of 1850 to 1900 fps. I used IMR4198 to start with in the 45-70 but was able to improve the velocity variation some by using RL-7. Have better luck with the IMR in my 458wm. But overall looks like you did some good testing and could pick out a good load form this data once you have done some accuracy testing. Good luck.
Domenic
Domenic