Create a basic Formula class, and a stubbed-out child Formula.
Add hook to intake's didSet property method to recalculate.

This adds a hook where you can define all your calculation logic in your
subclasses by overriding recalculate(), which I'll do next.

This method returns nothing, since it changes the state of the instance
of the formula. If you want to get the formula's new values, ask for
each of them individually after calling recalculate().