Depends on planes really. In the Ki84 I have it set to 225 yards for both cannons and MG's. This is about the range I usually shoot at though often I fire even closer.
For 50 cal planes, which I don't fly much, I have em set between 300 and 350, sometimes one pair at 300, next at 325 and the last pair at 350. Using the inner guns furthest out to make dispersion less. (Hardly any difference IF any).
190's I just brought the 20's up to 300 again, also trying 350 for them but think that 300 is max distance really for any MG151. While they can of course cause dammage at longer distances aswell the disperions is greater and usually a waste of ammo (more dependant on ones aim then on dispersion).
The discussion about nose mounted guns has always been a hot debate. Convergence does effect nose mounted guns, most notably in the height/trajectory.
Set convergence at 150 for the 109 cowl guns and shoot at 600 yards the bullets will spread out more then if you have them set to 600 yards convergence. However the difference isn't all that great and I sugest setting them to the same convergence as your other nose mopunted gun.
If you do have the cowl guns set to 200 and the 30mm set to 600 (or 20mm) there is a big chance of the cannon to overshoot your target if you're in close. If you set both guns to same convergence they will both hit "in the center" at that distance.
What the cannon (30mm specially) in the 109's could gain from a long distance convergence is that they aim a bit up at close distance, that MAY make some deflection shooting easier as you need to pull less lead but it will also force you to aim bellow targets close to you if they aren't pulling turns (such as a scissor fight where you take a shot when they roll over to the next direction).
I often see people saying "I have my convergence at 400-500". Think about this, how often do you yourself get kills at 400-500 yards in AH2? In AH 800 yard kills were common, very common specially from 50 cal and Hispanos. In AH2 the large majority of kills happen bellow 400, well bellow 400 often but people keep flying with convergence at 400+.
Also think about this, how often do you take 400+ yard shots and hit good? Not very often though it happens, I killed an LA7 from 600-800 yards with a 30mm shot a few weeks back, purly luck as I lobbed away a few 30's. Doesn't matter if you are right at target with 30mm at that distance as it will come down to dispersion, sometimes they may hit but usually not.
So, what distances do you often shoot at? Don't set your guns to kill runners or scare runners, set your guns to whatever distance you fire at most often and find that you hit the enemy most often. P51/P47 and other B&Z planes, set it further out as you may open up already at 400 or even 500 yards if you come in fast.
In turnfighters such as spits and ki84 set it in close, afterall a turnfight is often a very close combat fight.