The rate of twist depends pretty much on slug length, Casper, and some on speed, but that is of less importance then length.

The Greenhill Formula, when using the constant of 150 works for lower speed projectiles -up to about 1,500fps or 1,600fps.

The number derived from the Greenhill formula is the 'slowest' rate of twist for that length projectile, not the optimum rate.

Or, you can go here.
http://kwk.us/twist.html