Depending on the bullet you are using, you are pretty close on the calculation.
It will really depend on the center of gravity of the bullet you are using. A simple test is to just drop the bullet from an out streached arm point down and see how it lands. If it lands nose first it's center of gravity is toward the front of the bullet and will need less stabilization. If it lands tail first, the center of gravity is towards the rear of the bullet and will need to be pushed faster and can use a faster twist rate than one that is nose heavy.
As the bullet travels through the air the center of gravity is pushed faster than the lighter areas of the bullet, so the best bullet is one that the center of gravity is at mid point of the bullet.
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A bullet with the center of gravity shifted far to the rear will require much higher spin rate to maintain nose-forward attitude compared to a bullet with the CG shifted further forward. Most bullets have their center of gravity somewhat aft of the linear midpoint. If dropped from a height with the nose pointed down, they would land base first. A nose-heavy bullet, on the other hand, would land nose first. In flight it would require less spin to maintain nose-forward attitude. With the CG moved too far forward, the bullet would require little or no spin to strike nose first, but would follow the launch attitude over its trajectory path (that is, the nose would continue to point in the direction and angle at which it left the barrel, so that as the trajectory arc became larger with greater distances, the bullet would tend to fly at an angle to the direction its nose pointed, and would begin to tumble).
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