From testing the BK5, the maximum range for accuracy in the air is 2.5k and ground is 3k. But, you can hit the rearm pad at 5k about 1 in 5. At 3k I was hitting close enough to my helper sitting on the ground, about 3 in 5 to damage his plane. The dispersion sucks past 3k.
I'm thinking for practical reasons 4k for the 75mm. Many players I have watched from the 5inch forget to lead the ships by about a half ship length at 4k and farther out. By 3k in a B25H, they become cannon fodder unless they are already veering off to setup another run. But, starting at 4k, judging aim is possible with a reference point. 4.5-6k is all hail Mary shooting and a waste of ammo.
I have tried to make range markers for buildings, AA pits, and tanks. Because the game is a 3D world limited by the size of your screen pixels. A building or AA pit between 2.5 and 1.5k becomes the difference in one or two pixels based on 1Mil = 2pixel. They look the same. Like WW2 it really comes down to practice except when it comes to something as large as a ship. The 75mm in the PTO was used for it's blast damage to an area when attacking ground positions at low alt, 1500-2000ft. Not for trying to hit a 20ft wide speck we call a GV from 1000yds or farther out.
At the end of the war when the B25H\PBJ-1H was being used to attack supply shipping in the PTO. (Not like our game where we attack war ships.) And the pilot was limited to a manual gunsight with a 14 degree dial to raise and lower the reticle for range. The copilot read off dial settings while estimating range for the pilot. At 4000yds the hit rate was 3-4 out of 12 shots. When radar aiming was introduced which auto tilted the reflector plate. The hit rate at 4000yds went up to 9 out of 12. It was only possible because of the mass and size of the metal ship returned a strong signal. That happened at the very tail end of the war with only a few missions flown before the Japanese surrendered.
