The design program tells you the twist required for a stability factor of 1.5 at 1600 fps, using the Miller rule.
-- it is not a minimum suggested twist. A minimum suggested twist would use a stability factor of 1.3, not 1.5.
-- it does not include the length of the gas check, or any additional lengthening caused by sizing or firing the bullet.
-- when in doubt, consult one of the online Miller Stability calculators for your barrel's twist and your load's velocity.
-- none of the stability calculators may be reliable at transonic velocities.