Any measurement from the ground to a point on the rear wing will be influenced by wheel & tyre size.

A more consistent measure is between the high point of the wheel arch to the wheel centre.

But even this includes variable rear wing arch curves.

Peter Ballard's idea of the distance between the bottom of the axle and the chassis stop is probably most consistent, but not so easy to measure.

DaveW
'05 Red Roadster S1
'16 Yellow (Not the only) Narrow AR GDI Plus 4