Pglasgow

I think that formula deals with "minimum required twist" as opposed to "ideal twist" The greater the sectional density of the bullet (length as it relates to bore size), the more twist required to stabilize the bullet. One needs a twist number less than the minimum required for stabilization.

Regarding the velocity, that is a good question. Of course the faster the bullet the faster it spins. But as a bullet slows down ... so also does its rate of spin. Also the moments which would deflect the bullet are much greater at higher velocity thus requiring higher angular momentum of the bullet for stabilization. Though I do think velocity does play a role, I think the formula works fairly well for any velocity one expects at the muzzle. Basically because the same twist imparts sufficient angular momentum regardless of the muzzle velocity. If the muzzle velocity is higher ... then it needs greater angular momentum to be stable, however because it exits at that higher velocity it has the greater angular momentum it needs.