This is how I do it,
Set my ride height,
Then take my ride height + droop that I want, lets say 6mm and 2mm respectively , and add them together, so 8mm,
Now I take my vernier calipers and set it to 8mm, so now the depth measuring part of my verniers is sticking out 8mm,
Now I put my car on a level surface with the chassis hanging over a little bit at the front or back and lift the chassis from underneath at 1 end until the body of my verniers is level with the board the car is on and adjust my droop screws until the tyres just leave the board, and now my droop is set to 2mm.
I also set my tweak at the same time using this method.