Lets start from the beginning. I don't want to insult you so bear with me. RE/RL 15/17 are the powders used in the shell. I doubt any factory rounds will tell you whats in the shell. Most have proprietary powder blends - and they ain't telling whats in it.
If you want to reload for your 375 R, I suggest starting with something bland to get used to the gun, break it in, etc. I've had 2 - 375 Rugers and started with Varget under a 300 gr Hornady. You could use a 270 of about any flavor. I use 66.0 of Varget, 300 gr Hornady, and WLR primers. In 2 - 375's that load has been MOA or less. Recoil is very tolerable.
You can look at the Alliant website if you want to use the Reloder series. I didn't see any loads for the 375 R but they do list loads for the 375 H+H. Call them and they'll guide you to a good starting load.
Depending on your intended use, I'd match the bullet to the game. I likely wouldn't use a 270 TSX on deer although it'd work. Many folks use the 270 TSX for African game, namely Buffalo. If you want to use your 375 on something thin skinned, I'd ask the folks here what they use - I've never killed anything with either of my 375s. For NA thin skin big game, I think it would hard to beat a plain ole Hornady.
Hope this helps.