How's this?

for a .30 cal. 180 gr. Spitzer bullet that is 1.35 inches long. here is the formula.
150 divided by length of bullet in calibers. Only applies to bullet with specific gravity of 10.9- jacketed rifle bullets.

1.35/.30= 4.5 calibers; 150/4.5= 33.33 calibers, or that many times the diameter of the bullet. 33.33 x .30= 9.9999 or 1 in 10" twist.

This came from Mastering Mule Deer by Wayne Van Zwoll, and was referred to as the Greenhill formula.