So I built a flak cannon but as you may know you cannot change ranges so I made a line up of them each shooting at different ranges
And then one in the back for more long range shot
Ag1 to activate that one

So I did a bit of range finding too
If you go into the XML modifier you can change the ranges
(I like these ones because AI likes to fly closer to attack)
Anyway if you change the projectile lifetime then I changes the range
For example:
Projectile lifetime on 1 will get you 900m
If you add another 0.5 it will give you 1100 (it shoots anther 200m)
And if you do the math correctly then you can probably figure out the rest

Disclaimer: these measurements are not 100% correct and may change in an update

Oh yeah!
Trim to turn the cannon left and right
Pitch to aim up (don't aim down! :) )