If you have difficulty to see if the x_end_idler is parallel to the frame you can use a thick ruler or a square like on the first image.

Verify that your Z lead screw is not touching the Z tops holes. If it is touching release the pressure from the X smooth rods tensioning screws like explained in the previous step.

Note that Z lead screw does not need to be centered totally, it can wobble a little.

Double check that your belt is still well tensioned. In case it needs adjustment repeat the previous step.