Starting with the firmware version 3.0.12, more detailed information about XYZ calibration results becomes available. This feature can be found in LCD menu -> Support -> XYZ cal. details.
The first screen tells you the distance of the “perfect” position of your front 1st, 2nd, and 3rd calibration points. Ideally, all of these are positive and at least 0.5 mm or more. If the margin is between 0 and 0.4 mm, then there is a chance that your calibration will not be successful ("XYZ calibration failed" error).
To achieve the best results, aim for the smallest possible difference between Left and Right numbers. A difference of up to 0.1mm is considered good.
Pressing the knob will take you to the second screen. This screen identifies how far you are from the perfect perpendicularity. It is measuring the skew of your X/Y axis.
It may seem that the compensations are not that significant. However, if we take into consideration the 250 mm X-axis length, 1.1 mm is a large margin. In order to improve your axis perpendicularity, make sure the difference in the front calibration points distance is minimal.
When you get your axes perpendicular or just slightly skewed, nothing needs to be tweaked as the printer will perform with the best accuracy.