I have both a .25-06 and .300 Win. Mag. and would like to simplify things by using one powder for both. I looked in my Barnes #2 Manual and it looks quite efficient. According to my calculations...

.300 Win. Mag.
200 grain XFB 64.0 grs. H4350-2700 fps.
.25-06
115 grain XFB 47.5 grs. H4350-2900 fps.

Does this sound about right?

I sold my .300 Win. several years ago, but my rifle preferred H4831 for the heavier (180-200 gr.) bullets. H4831 also works great in my .25-06, but IMR-7828 works even better. IMR-7828 should also work well in the .300. You might also consider RL-22 for both rifles. H4350 seemed a little fast in both my .25-06 and .300 and does not fill the cases as full. I use Nosler partitions bullets. Maybe the faster powders work better with the Barnes X bullets you plan to use. I would recommend trying several powders before you buy a large quantity of anything.

The 4350-range powders are a bit on the fast side for the .300 magnums and also the for .25/'06. They can be used, but are less than optimum.