That really only comes into play with heavy lead deflection shots.
For 125 yards, for 525 yards, it's automatically set so that the wing cannons and the nose MGs hit the same spots. The only reason they would have different convergences is for shots you make while pulling Gs. If you've got a good steady shot it doesn't matter.
P.S. IMO forget "lifting" the cannons. You want to shoot higher, AIM higher. You'll get a better mental picture and you can aim wherever you like (often times when I land hits my "center" is not on the target -- its like flying looking backwards, something you should be able to do)