I've had them but never used them as intended.
There was a calculator on their website where you would input your ballistic data and it would give a specific range (distance) for each of the hashmarks on the reticle, but it was computed at one specific magnification (which may or may not be marked on the adjustment ring, e.g., 12.5X).
You could go out and shoot known distances and see where the impacts correlate to the reticle, but again it would only work at one magnification (that's how I used it).
Too many better reticle options these days to mess with it. Even the Zeiss ZMOA is better (hashmarks are 1 moa at max. mag., 2 moa at 1/2 in the magnification range, etc.).