Daryl, I also input velocities every 200 f.p.s. slower, down to 1,300 f.p.s.
1,300 f.p.s. gave a Stability Factor of 1.382, still in the green.
At 1,160 f.p.s. the Stability Factor is 1.302, barely in the green.
At 1,155 f.p.s., the bullet becomes unstable.
My experience with bullets in a muzzleloader is limited. I would think that from the information obtained from the JBM Stability Calculator, that I would want the bullet to be moving at at least 1,200 f.p.s. upon impact? Maybe faster?
Before I would shoot such a bullet at a big game animal, I would want to spend considerable time at a range with a chronograph, at distances out to 100 yards, with the results repeated at least 20 times, until I was satisfied that my powder charge, wad, & bullet were absolutely capable of repeatable results. And, that I was capable of shooting said load ethically out to the maximum distance I felt comfortable with.