I also shoot an H&R Handi-Rifle in 45-70. My 45-70 load is almost exactly the same as yours: H4198 with 300 gr. Hornady jacketed soft-point bullet. I worked up loads; 5 each, 50, 51, 52, 53, 54, & 55 gr. and shot them over my chronograph.
When I got to the 55 gr. load, my average velocity was...