Chester;
Ken Waters in his book of "Pet Loads" list the following loads as being very accurate in their 257 Weatherby rifle.
1. 67 grains IMR-4831, 100 gr. Speer SP. 3436 FPS OAL-3.18
2. 65 grains IMR-4831, 100 gr. Sierra SP,3187 FPS OAL-3.20
3. 66 grains H-450, 117 gr. Sierra flat base, 3137 FPS OAL-3.20
4. 64.5 gr. H-4831, 117 gr. Nosler Part. 3140 FPS, OAL-3.20
It seems that IMR-4831 is a good powder to start with. The load with 67 grains of IMR-4831 was a near Max load in their test rifle. You may want to start 10% lower and work up. Hope this helps you.
RELH