I noticed that the Mi-48 gunner has the option to set the distance - just use the default keys for that. But there is no indicator for this setup.

Therefore you're right and it should be fixed.

To quote myself: "If you adjust the main optic on the weapon, you don't move the complete optic. It just moves the reticule pattern inside the optic."

Correct me if I am wrong. But nowadays you don't move the complete optic if you adjust them. That would be far too inaccurate. For example if you let your weapon fall down or something like that, the whole adjustment is gone.