If I'm reading right, you're calculating for a steel ball. Isn't 500 fps a little low? What charge would give that velocity?
Yes, that is correct. It is an entirely WAG and I have no idea how much powder would be required. However, a typical gonne has a relatively short bore compared to a more modern cannon so I would not expect the ball to achieve similar velocities and, since the gonnes are typically chambered, I wouldn't expect the charge to be as great as a cannon.
Time for some measured experiments.