I generally push the car down once to settle the springs, measure that height from the ground, then with my baby finger pull the end up just to the point where I can feel it getting heavier to lift, when the tires want to lift up. Measure that height, and then subtract the difference in height. To make it even more noticeable, you can weigh the tires down a bit, but not too much to deform the tire.
Make sure the droop screws are even before doing this, so you get an accurate measurement across the chassis.
I have a small metal ruler I line up against the bottom of the chassis, which also gets the ride height, but I guess as long as you find a common spot to measure from it's all good. This method works for all tires, regardless of size. Are there any other methods?