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

QUESTION:
In the screencast below, note the equation of the pink lid is of the form z = some constant. How do we mathematically determine what this constant is?
Quick Demo
– Tim Brzezinski

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