Sorry for the late response-
I do not think there is a minimum height difference for auto drain valves. One other thing I forgot to mention- On systems with master valves, or ran by pump, we install a drain valve on every manifold so they drain out every time. We then intstall a boiler drain before the master valve, but after the main valve, so we can drain that part. Then, we put autos on the end of the laterals and mains, and also where a low spot is where water may collect. This eliminates blow-out of the system, but you still have to do a little winterization:
1. Shut off main line and drain the line between the main valve and master valve
2. Turn controller to "OFF" mode, or unplug it and remove battery-which ever one the users manual says to do.