The way I measure droop:
I first set the down stops with a droop gauge (this varies with tire diameter), I then set my ride height. Then I use my ride height gauge and push it under the chassis till my tires are off the setup board. I slowly pull it out so that the tires are just touching the board. The difference between that height and the ride height is my droop. For a very smooth technical track, I try for 3/4mm of droop.