Hi mate , its a question dear to my heart as I am also shite at calculating gunnery. I have studied it and put a collection of tips here, including ww2 RAF, Luftwaffe and USAAF docs
In BOX I have my convergence at 300m and give a lot of lead, if the bandit is turning at all I am basically shooting blind under my nose. Dont chase the pipper, try to learn how many rings lead for the angle the bandit is off. Getting in close is good.
The FW is very sensitive to changes in Angle of attack (or pitch) try to fly as smooth as possible and set up your attack , best practice is you shouldnt be turning in it , dive, turn no more than 20 degrees then wings level and zoom up again
Hope that helps