Earlier this week someone was talking about using the vertical targeting option. I didn't exactly understand it's use until I found these posts. Hopefully they are clear. If you've ever sighted in a rifle, it will be easy to understand. http://forum.warthunder.com/index.php?/topic/170385-does-vertical-targeting-really-work/?p=3205364 http://warthunder.wikia.com/wiki/Gun_Convergence Sorry, I don't have a long explaination for this, but the attached articles and pics are pretty good.
Eventhough this thread is about vertical targeting, you will want to consider the effect of horizontal convergence in how you set zero. When I was reading this, I thought man, veritically, if I set my convergence (or zero) at 200m, I should basically be shoooting flat for about 500m. But remember the horizontal effects of setting convergence to 200m. Basically you will have concentrated your guns very close to your nose. If you need to shoot at distance, you'll have a heck of a horizontal spread at 800-1000m.