Buy exbal ballistics program and once you have a load, you can use the reticle tool in the program to get specific reference points for each crosshair and at different powers. that is no different for a crosshair scope. Same steps.
All of them have to be rezeroed if you change loads, so no difference there.
In reality it is no different than a crosshair scope. You have to have good ballistic data and a dope chart. Only difference here is you have dope charts for several power settings.
It is really easy to do and in reality it is just as easy to use past 500 as a fixed crosshair, especially if you have the larger zooms up to 800 or so. You can have simple tape on side of stock with the power setting and zero ranges for each crosshair. After that it is simple click up like a regular crosshair scope.
Once you know the range to the target, look at the dope card or tape on side of stock and see what power and crosshair is closest to your given distance.
On a Kahles 3-9X, my max distances between are no more than 60 yards so you have just a minor hold over or under impact point. I zero at 200 and can go from there to 840 yards at no more than 60 yards between crosshairs I have used them on game at over 500 using that rifle and hit targets at over 700 regularily. I have set up a 6-18 TDS and and the 4-14 BC to do the same.
I have a 338 Lapua AI now with the 8-25 LR with varmint reticle and it will have no difficulty going way past 500 on the crosshair data and I can click up if I want to.
BH