You are more or less correct, I don't use the crosshairs adjustment (this was a feature missing from IL1946 as well as being innaccurate at the moment (I just move them to the edge of the ring and out of the way). I use the relative size of the enemy ac as it appears in my gunsight ring. A BF109 that appears to be half the size of the diameter of my ring is approx 200m away. My guns are converged at 150 so at half size it is just a tad further away than ideal. A stuka would be approx 2/3 of my rings diameter at optimum firing point, you get the picture I am sure.
Your ring has two crosshairs adjustments, one for your guns current convergence and a second adjustment to set the crosshairs to suit the wingspan of a particular ac. I have used zonal convergence for ground attack missions against slow moving or stationary targets - I wouldn't use it in clod against air targets though but try it ans see how you get on.
EDIT:
This video may help