I spent a lot of time looking for this information here, but was unable to find any cast loading data for reduced 375 H&H loads. I struck out on my own, and here are my findings.

Hope this is of benefit to someone else.

Brass: Winchester
Primers: Fed 215
Powder: RL7
Bullets: 335gr Cast w/GC
Filler: 1.5gr of Poly-Filler
Chronograph: Oehler 35P

35gr = 1715 fps (average)
37gr = 1760 fps (average)
39gr = 1830 fps (average)

All groups were under 1.5" at 50 yds (open-sighted)

You might try 14 gr of Unique, should drop you down in the 1200-1400 fps range.

I use 14 gr of unique in my 350 Rigby (almost same capacity as the 375 H&H), it drives my 200 gr cast bullet 1400 fps, with an es of 20 fps for 10 shots, and groups 5 into 1/2" at 50 yds. I use CCI 250's with no filler. I need to try the same charge with a 280 gr cast bullet.

Unique and 2400 are outstanding for mild cast loads in rifles. They ignite consistantly with no need for fillers, and provide excellent accuracy.

If you do a search using my user name <andrew375> you will find several posts on the subject.