RL-15 was my old favorite in the .375 H&H with 250-gr Sierra GameKing
and even worked great with the old 300-gr X-Bullet.
The latter is what I took to Botswana in 2001.
Nowadays I would prefer Varget,
or CFE-223 as suggested by jwp475.
CFE-223 will likely get you higher velocity at lower pressure.
Good luck finding powders nowadays.
Any of those three should do.

Ron aka "Rip" for Riflecrank Internationale Permanente
NRA Life Benefactor and Beneficiary
.458 Winchester Magnum, Magnanimous in Victory
THE WALKING DEAD does so remind me of Democrap voters. Donkeypox.