Gunnery/Sights question

[noxnoctum]

I haven't been playing IL2 quite as much as I used to and my gunnery has suffered dramatically. I used to be able to just eyeball any shot basically but now coming back to it I would like to be using the gunsight to its fullest potential. I've read some that the different lines represent angle-off of your target (for example outer ring on 109 gunsight is 20 degrees I believe) but just wondered if anyone has more concrete info on how to use the sight properly.

Thanks!