Hello M&M,
I would suggest using any of the following recommended reference manuals:
Lees Reloading Manual
Lyman Reloading Manual
Hornady Reloading Manual.
We have many recommendation in the forum for 45/70 loads, but it's best to understand the standard loads in these manuals before venturing into custom loads. So one thing to remember when looking up the standard loads, slower is better to a point. The happy place is between 1160 fps and 1350 fps with a 350 - 400 gr bullet. The energy delivered by the Marlin 444, 45/70 is mass times the velocity.
So a bullet weighing say 400grs traveling at 1350 fps is delivering enough energy to take out the most dangerous game world wide. But, you must do your part in shot placement. So if you hit a cape buffalo between the horns where the pounding of rivle animals is done, your bullet may just bounce off. Then you have a pi$$ed off buffalo, that may not end well....:flute:
Have fun enjoy your new rifle and Welcome from South Carolina.