There is no black hole that is a perfect sphere. That would, at a minimum, require a body with absolutely no angular momentum which isn't in anyway feasible.
Any rotating/spinning black hole will no longer be a perfect sphere.
But even then, the biggest black hole we think is possible measured down to the planck length gives you a number with 50 digits. And the entire observable universe measured in planck lengths is about 60 digits.
So how are you going to get a physical pi of even a hundred digits on the path toward arbitrary precision?
Any rotating/spinning black hole will no longer be a perfect sphere.