When I talk about deflection shots, I am referring to leading your target. The only thing that I can say will truly make you better is to practice. I don't know of any fool proof way of determining how much to lead your target. I do know that SimHQ has a few nice articles on this topic by Andy Bush. There is a three part one:
part 1,
part 2, and
part 3. There is also a
further article where he talks about convergence, sights, and such. They are very good reading and will give you some insight into how best to approach a good gunnery run.
As for me, I tend to just go by feel. The sight that I use is a simple dot. I used to use one that was a historic sight with all osrts of rings. I quit using it because it would just get in the way of a good view of the target. The more I have been flying this sim, I have gotten a better feel for target aspect, relative speed, and how much I need to lead my target. I don't know how to honestly explain it, but I do go through a very quick mental evaluation of what is before me so that I can get a good shot off. It comes with practice. The more you do it, the easier it becomes and the less you will miss. That being said though, there are nights where I can kill 6 planes and be forced to RTB due to fuel shortage even though I have half my ammo left. There are other nights (too many IMO) where it takes me more ammo than I care to admit to nail a single kill.
As far as convergence goes, most people I know have theirs set to 400 and below. I am the only one in my squad, I believe, that has convergence set to 500. I like it because it aids me in the snapshots more than anything else. I can get a nice spread and nail the wings of my opponent. The best thing that I can recommend about this is to set it to something one night, and keep it there for a few days or weeks and see if it works for you. See what distance you are getting kills at most of the time and set your convergence to something in that ball park.
Hope this helps,
-math