I finally manage to setup the ABL on my Prusa. I run G28 to home all axes (for z I use the same probe used for ABL). Then I run G29 E ( I use E because otherwise for some reason I have not understood yet, Z does not lift during probings) probing 4 points on the corners of the bed.
finally I get the map of the bed:
Bed Level Correction Matrix: +0.999999 +0.000000 +0.001233 -0.000005 +0.999992 +0.003905 -0.001233 -0.003905 +0.999992
and after that, the print starts. The first layer looks perfect however I have not seen Z moving a single step along the printing.
Any hint about what to check? Is the obtained map indicating that the bed is already too leveled to act on any compensation?
UPDATE I printed a 180 mm diamater cylinder and the Z axis is not compensating the 1mm difference from edge to edge of the bed.
I believe that the matrix is shown transposed from how it should be, but that doesn't affect the answer. The compensated Z position is derived from the original
(X,Y,Z) position by multiplying the corresponding vector with that matrix. This means that the new Z position would be
Z' = 0.999992Z - 0.001233X - 0.003905Y
If you have a 200 x 200 printbed, a diagonal move from one corner to the other would correspond to a 1mm change in Z-height. This should be noticeable, but if perhaps you're printing something quite small you might not notice it.
Though, if your first layer is perfect, I would see no reason to mess with it.