The case length of the .375 H&H and the .375 Weatherby are essentially the same at 2.85" and .2.86" respectively, so I wopuld simply load to the bullet cannelure and call it good, provided that length will cycle through your magazine without hanging up.
.375 Weatherby 300 gr loads from the A-Square manual:
IMR-4350 start 80 grs for 2546 fps up to a max load of 89 grs for 2709 fps. 85 grs gives you .375 H&Hish performance of 2641 fps.
IMR-4831 start 84 grs for 2620 fps up to a max load of 88 grs for 2730 fps.
Re-19 start at 88 grs for 2558 fps up to a max load of 92 grs for 2745 fps
Re-22 start at 88 grs for 2598 fps up to a maximum of 91.5 grs for 2694 fps.
My observation of this data, for whatever that's worth, is that the relationship as I understand it between 4350 and 4831 and between Re-19 and Re-22 is somewhat ignored by this data. But perhaps Art Alphin was attempting to produce lower pressure loading with he slower burning powder loads rather than attempting to get the maximum performance possible. In any case, work up your loads carefully, and check the velocities against what was published as you go.
