With the 37mm you can set your zero to 1000 using the offline target and no TR or VR enabled. Use the page up and page down keys with the target set for 1000. In real life guns like this were set to about 1500 or 3000 for zero and you learned to adjust. How it's set in the game, 200-800 is about halfway down the center spike inside the center ring. 1000 is just above the first radi lead ring. The rings are not for range but instead for lead calculation while the gunner watched the tracers. If you set the bottom of the first ring to 1500, then half way down between the first ring and second is roughly 2500. Small arms gunnery like our 37mm was a matter of skill and experience like learning to skeet shoot.
The Outside of the iron ring is 140Mil and knowing the m\sec of the round, then calculating for 250, 300, 350, 400mph. You can create a lead chart at range. AAF gunners had them while talking about lead as number of radi of the gunsight. And how do I know this, learned all of it while doing the years of research that resulted in the AH3 historic reticles in the game.
Find a copy online of the Aircrewmans Gunnery Manual. Find a manual or just the ammo specs for the Flak 36 3.7cm. Past 1500 dispersion makes it hard to hit planes smaller than a bomber.
Oh, it's not called calibration it's called "zeroing" and you could always see what it was at any time offline using the target. Waffle seems to have chosen about 400 for it's zero. After that your problems are speed of the con, lead, under or over for angle off and dispersion at range. It's a wonder anyone hits anything in this game with one of those at 120rnds\min. To bad Hitech didn't give us the twin barrel Flak 43 or update the gunsight.
You want to be able to zero your 37mm or, turn on the lead computing sight and shoot at a drone from 800-1500 away. Place the offline terrain from this download in your ah3terr directory, it only works offline. Fill the drone slots with planes and pick the most south 37mm which will spawn you on the mesa above the field under the drones. Spawn a wirble up there or fill the drone slots with tanks and spawn N, E to shoot at them long range at the tank gunnery ranges. Keep in mind the drones are flying by at 250.
Download: ====>
https://drive.google.com/file/d/1i3N1PhAckNls_53ZA4hHPk5UnGK4jY-W/view?usp=sharing