Not all X axis have the exact same length because of tolerances (including on Original Prusa printers). This step will adjust your X axis length.

The 1st image shows what happens to the X ends when you tension the X axis belt. We want to have the X ends as parallel as possible to the frame (or it will affect the X belt motion).

1st image, top left: when the X axis is too short.

1st image, bottom left: when X axis is too long.

1st image, right: when the X axis has the perfect length.

To adjust the length of the X axis we need to use the two M3x10 screws of the x_en_idler and look at the Z tops.

While you tighten the M3x10 screws keep an eye on the Z tops. The M3x10 screws will move the Z leadscrews laterally (left or right), adjust them to have the Z leadscrews as possible centered.

Note that the M3x10 screws will not move back and forth the Z lead screw position, only laterally.