It seems like a simple thing to set up.
Here's what I'd do:
1: Target holder would be very tall and narrow. Sufficiently tall to handle the drop @ 600 from a 100 yd zero.
Zero at 100.
Move the target from 100 to 600 shoot at each distance using the zero hold.
You will then know the approximate drop at each distance.
The natural instinct would be to then say the reticle would be calibrated.
However, the next thing would be to shoot at those distances again using the reticle hold over stadia. It was explained to me that this would remove the error due to the cosine of gravity.
Also see the following for much better explanation: http://www.longrangehunting.com/articles/long-range-targets.php