In the old FWIW column
I pulled all of my reloading manuals that I have in print, looked at what powders were used across Barnes #4, Nosler #8, Speer #14, and Hornady #9 for the 300 grain 375 Holland & Holland Magnum bullets that were listed. I then compared this information to my on hand inventory, as well as consulted a very well respected member here on AH offline of the forums.
Further I then discussed what was being used powder wise for 300 grain bullets with the chief gunsmith / rifle tester / ammunition loading lead at the shop that is building my first 375 H&H, Parkwest Arms. I should say that I have a degree of experience working with other well regarded builders for other platforms that I have had built, in several different calibers, with respect to initial loads and reloading for accuracy at distance.
At the end of all of these fun and games, Varget percolated up. And as it turns out, Parkwest will be using two powders when finishing up, testing, and sighting in my build: RL-15 and Varget. However, It's only found in one of the four manuals I consulted, Hornady #9. As I am sure most are aware, Varget is a highly regarded powder for other cartridges.
Consulting this burn rate chart, RL-15 (105) and Varget (108) are close to one another -
https://imrpowder.com/wp-content/uploads/2019/12/2020-burn-rate-chart.pdf
Not trying to "sell" anyone on Varget per se, but it's in my inventory. Perhaps running a similar compare / contrast exercise against your inventory would be helpful? And I am certainly not trying to invent a new AND improved load for the 375, just looking at viable alternatives. Varget may very well work for me due to inventory, and dumb luck has me stumbling across a powder used by my gunsmith. Once my SD-76 is complete, I’ll share the details of my load development.
And I’d be remiss if I didn’t add that all the standard, non-standard, applicable, asterisked, footnoted blah blah blah disclaimers apply – work loads up from published and reputable reloading resources. I also use QuickLoad for all my reloading work.
Lastly, I are engineer (ha!), and this was all highly unscientific up to this point.
Matt