Somehow I didn't hear about the Miller Twist Rule until just now. Oh well, better late than never.
Anyway, it's a spinoff of Greenhill's formula, and sometimes used by benchrest shooters who are looking for the minimum twist to stabilize a particular bullet at a particular velocity. The general rule of thumb is that best benchrest velocity happens when the bullet is spun just fast enough to be on the ragged edge of stability.
I slapped together a spreadsheet to calculate the required Miller's Twist Rate for some loads that I have been playing with lately, assuming a stability factor of 1.2 (less than 1.0 is unstable, short range benchrest favors 1.2, long range benchrest favors 1.4).
155 gr. spitzer in 357 mag at 2000 fps: 1 turn in 25.4"
60 gr. spitzer in 6x45 at 2500 fps: 1 turn in 17.7"
135 gr. spitzer in 30-06 at 2500 fps: 1 turn in 20"
165 gr. spitzer in 30 caliber at 2500 fps: 1 turn in 17.7"
Compared to conventional wisdom and practice, those are some pretty slow twists.
Update: Here is an online calculator for Miller's Stability factor. You tell it your bullet and twist, it tells you the stability factor. Here is the output for my Contender 357 mag: