Jasper, Where are you finding a tab for Mode S Max Range ?
edit: Think I read your post wrong..
Ans: that icon displays what if you have the best conditions the limits of Mode-S signal, remembering anything between you and any given direction will reduce your overall range.
Much better, Once you've used the RB for sometime click on the icon for Polar Diagram (one right of max range) and that will show you better your range by direction.
Another thought, when you click Max Range you see nothing on screen, zoom out and you'll see it places two circles at you max range.