I would chrono the factory loads and stop when you reached that velocity. R15 has been my go to powder and is less bulky than 4350. I always ran out of room with 4350 and heavy bullets before top velocities were achieved, but this is some what a moot point as the rimmed 375 and others have performed well at lower velocities for decades.
Another when I get around to it project is to re-work my 375 loads with R16 for less temperature variance and maybe more velocity. R16 is right between 15 & 17 but almost a duplicate of 17. It is denser than 4350 so I hope it will work out it should.

"When you disarm the people, you commence to offend them and show that you distrust them either through cowardice or lack of confidence, and both of these opinions generate hatred." Niccolo Machiavelli