All I do is measure from the lip of the bottom collar to the adjuster knob, by putting the end of the tape measure on the knob so it holds it while then stretching out tape.
I wouldn't measure from mirrors as these are further away from the ends of the dampers so any discrepancy will be magnified, even more so when the mirrors are different heights each side due to droop mentioned.
It is best to get the car corner weighted, as this will produce the best handling grip for the car, which is why you have modded suspension.
If you get the ride height "correct" it goes out as soon as you get in anyway!!
Get it corner weighted with you sat in the seat.
tabetha