I don't own a .375 H&H Magnum, so defer to Nick Harvey's recommendations which are about right for this cartridge according to those of my friends who do have one.

Uncle Nick's recommended maximum load for the 300gr bullet is 81gr IMR4831 for 2640 fps in a 24-inch barrel.

He doesn't give a load for the 250gr bullet in the loading manual edition I use, but gets just over 2700 fps with several maximum loads for the 270gr bullet. One can therefore deduce that 2700 fps would be easily achievable with the 250-grainer.

All the usual warnings apply, 10% under and work up, etc.

BTW, where do you live, jro45? It does make a difference as to which powders and loading manuals other posters may recommend. Perhaps you could be so kind as to update your profile? I take it from your reference to 52 degrees that it is either somewhere in the US (farenheit) or else Marble Bar! (centigrade)!!