"Just out of interest how did other flight sims handle the Icon thing?"
When I played Warbirds (up to 2.77), the WarBirds ICONS gave range as a decimal to the closest 100 yards (such as 23, or 9). The range part of the ICON was visible at a greater distance than the PLANE TYPE icon. The Warbirds ICON system was better than AH's, IMO. I have no experience with the newer versions of WB's so I cannot comment on any changes they may have made.
AirWarrior originally used a very unique system. The ICONS weren't on-screen but, rather, were off to the sides. You had a hard limit of 12 planes which would display. You could not instantly know the E-states and positions of 30 different aircraft in the vicinity, AND you couldn't really use the icon like a "lazer range finder" for the purpose of aiming because you had to take your eyes off the plane in order to check the ICON. Although AW's system was cumbersome, I feel it ended up being the most realistic in terms of overall effect.
WW2OL apparently uses a system by which the ICON "greys out" as distance increases. It also seems to have a component which shows the lift vector of your opponent. I have minimal experience with WW2OL so this may or may not be completely accurate.
J_A_B