Assuming that you are talking about a Berger flat base 64, I have used that one and the former 65 grain for quite a few years. Is 500 yards a realistic distance, or are you thinking of a boattail design and wanting to reach out a little further?
If the former.
I would figure the muzzle velocity with your bullet of choice, and try to get as close to 195,000 to 200,000 RPM as possible. Use whatever twist is indicated, to achieve that goal. Can't find my formula, right now, but it's simple.
Good hunting. LB