Expanssion Vessel Pressure
If you have to fiddle with it every two months, the air bag/bladder/diaphragm is leaking. If there is any sort of guarantee/warranty, you might want to use that. If not, you might want to select a different brand for the next one.

As for the pressure, think of it as the lower end of the pressure you'd like to see supplied in the house when several things are drawing. So if you set it to 1 bar, pressure will drop to 1 bar. If you have 4.3 bar supplied and would like to have it drop to no more than 3 bar, pressurize to 3 bar. You'll store less water, but at a greater usable pressure. If you need to store more water, you need a larger tank, or several small tanks working together.

While you cannot accurately set the air pressure without draining the system, you certainly can just pump air into the tank with pressure on the system, particularly if you are not going to replace the leaky bladder, since setting the tank pressure is not a thing that will stay set while the thing continues to leak.