You measure droop with a ruler. How far up the chassis travels before the tires leave the ground. THAT is droop. What you are discussing, is the car's downstop setting, which is the difference in millimeteres between the bottom of the suspension arm, and the chassis. They are not the same thing.
Ride height = how far the chassis is off the ground
Downstops = distance from the chassis to the bottom of the suspension arm
Droop = how far the chassis lifts before the tires lift
Droop is a result of both ride height, and downstops.