Don't take any of this as gospel, but..... it helped me.
I use the Revi16d gun sight.
I pull up and start aiming where the arrow is (in the attached modified pic). Sometimes lower, depending on how many G's you are pulling. I have convergence set at d400 for the 30mm (even though it's a single cannon, it will affect the ballistics and where the tater drops) . I'll set mg's at d500, especially if I am flying with squaddies so I can spook a con off someone.
Obviously it is best to pull as little lead as possible, but the one being pursued usually panics when they hear that 108 barking. I have found that it is best to wait til you have a sure shot and squeeze off a few at a time. If nothing else flying with only 65 cannon rounds will teach you to husband your ammo devil
Also the 30mm rounds usually had 41 m (135 ft) of drop in the first 1,000 m (3,300 ft) of range. WHOA!
So.... in a perfect world in perfectly level flight, using a perfect angle of drop (not accounting for wind, or arc of drop as it would not fly at a perfect downward angle, or other adverse effects on ballistics) it would go like this.
d200 = 8' - 2" of drop
d300 = 12' - 3" of drop
d400 = 16' - 4" of drop
d600 = 24' - 6" of drop
the angle of descent is (2) degrees or roughly 4' of drop for every 100' of distance (again a perfect angle, not a true ballistics arc)
Taking into account that a B-24 Liberator is about 18' tall, you have to just about point the center of the pipper OVER the buffs to actually HIT them with taters at d400 WHOA!
Also, and observation from a squaddie...........
But you forgot that when setting your convergence it also sets it vertically, so if you set the MK 108 to 300 yards it will hit in the center of your gunsight.
Hope it helps..........