I have a 300wsm instead of 300WM, but anything over 75% of max charge of H4895 gives me decent groups at 100m (~1MOA, using campro, so my guess is the bullet is what limits the accuracy).
The loads published by Hodgdon is 51 to 54.5 grains (velocity is 2919 to 3018fps). They suggest to use 60% or more of the max load of H4895 for reduced loads.
If I use 34 grains of powder (62.4%), I obtain a velocity of 1951fps. The drop at 100m is insane and the group is big. I don't consider this a load that "works".
If I use 40 grains of powder (73.4%), I obtain a velocity of 2213fps and 1.5MOA 5-shot group.
If I use 45 grains of powder (82.7%), I obtain a velocity of 2508fps and 1.3MOA 5-shot group.
H4895 seems to be THE powder for reduced loads. At least for small range. The groups might open up at longer range.