not going to get the highest possible velocity, but 4064 is a great powder for the heavier weight bullets. it is the reccomended accuracy powder in several manual, plus it's usually easy to get a hold of.
i don't have a 22-250, but i was just researching it the other night to see if it's something that i would be interested in. it seems like a pretty great cartridge and works with most of the components i already keep on hand, i would like to get a bolt gun in 250 some day. you'll just have to try a few to see what works best for you though