Somehow I didn't hear about the until 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: 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: