Marlin firmware: angle calibration


I usually need very accurate parts and I calibrated my printer VERY well in X, Y and Z.

To calibrate the angle on the XY plane there seems to be no way of doing this in marlin or am I wrong!?

In other words: If an L shaped object does not have 90° on the corner there is no way to adjust the angle via a value in the firmware - I have to adjust the belt/slider to archieve those 90 ° (which is not that easy as I have to loosen both gears on one side and re-adjust them).