Styrofoam Coffee Cup (with Lid) Modeling in GeoGebra Augmented Reality

QUESTION:
In the screencast below, note the equation of the [b][color=#ff00ff]pink lid[/color][/b] is of the form [i]z[/i] = some constant. [br][br]How do we mathematically determine what this constant is?
Quick Demo
– Tim Brzezinski

Information: Styrofoam Coffee Cup (with Lid) Modeling in GeoGebra Augmented Reality