If I can explain on pops behalf...
The "true" measurement of droop is the difference between a cars ride height, and the height the car can be lifted to before the wheels leave the ground.
So, a car with 5mm ride height, and 2mm droop, could be lifted to 7mm before the wheels leave the ground.
You can use a ride height gauge to measure these distances, although my gauge only goes to 6mm so I tend to use a steel rule.