I love my Marlin 375. I use the Lee 379-250-RF sized to .377. The two loads I play with are:
Mild load - 25.1gr IMR 4198
Heavier - 26gr IMR 4227
The mild load, according to Lyman should be around 1600fps and the heavier around 1925fps. My mix comes out to 14-16 BHN as read by a Lee Hardness tester. Bullet drop is much more significant than higher vel jacketed, so anticipate this when sighting in.