Sounds like you have a good start on a load. Is the 84gr of powder a weight measure or did you mean 85gr by volumn?

I have been shooting a Magbolt for years now and have several loads that work well in it. Here are some of them.

348 Powerbelt with 100gr of 777 averages 1535fps and 1" or less at 100 yards.
250 Shockwave with 80 to 100gr of 777 averages 1600 to 1780fps and1.5" or less at 100 yards.
300 Shockwave with 100gr of 777 averages 1785fps and 1.5" or less at 100 yards.
300 SST with 130gr of Pyro pellets averages 1945fps and 1.5" or less at 100 yards.
285 Barnes with 100gr of 777 averages 1780fps and under 2" at 100 yards.
200 Shockwave with 100gr of 777 averages 1988fps and under 2" at 100 yards.
460 No Excuses with 90gr of 777 averages 1470fps andunder 2" at 100 yards.

I have other loads but that should give you an idea of the capabilities of the Magbolt. Many of these loads will shoot with acceptable accuracy with lower powder loads and some with higher powder loads. I have shot most of these loads at 200 yards and they generally group around 3 to 4 inches ifI do my part.

Good luck to your daughter and yourself in this fun adventure of muzzleloading.