dukes

Just get them to turn walk on and watch where they are. As soon as one of them charges, get them to stop dead and see how far they are, then you have a pretty exact measure of how far it is. The general rule i use is ~3-4 blaze lengths (the fire that gets dropped on the floor) or "stay inside the outer circle" although that's a bit less accurate.

Another thing that helps massively for tanking the elementals is to increase the max camera range. Someone posted it earlier in one of these threads (hard to keep track of where it actually was) - /console cameraDistanceMaxFactor 4