Spitfire Gunsight adjustment?

[Flashman]

I have been playing about with this but I dont think it works properly. What I do is this:

1) Set the sight to the guns convergence (remember its in yards, there are 110 yards in 100 metres)

2) Set the lower ring in feet for the wingspan, eg 32 ft for 109, 66 feet for JU88 (cheers wiki!).

Now this adjusts the size of the cross. It is my understanding that when the enemy aircraft wingtips are toughing the cross hairs inner edge, it is the required distance (i.e. the top ring) from you, i.e. in convergence.

However in testing with the icons (showing distance in KM) I find that the sight picture looks incorrect. I must stress I might be doing this wrong!

Anyway I will take some screenshots and post them so you can see what im doing.