Not all X axes have the exact same length due to manufacturing tolerances (this includes Original Prusa printers). This step will adjust your X axis length.

The 1st image shows what happens 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). (Note! the amount of deflection is exaggerated for illustrative purposes.)

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

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

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

To adjust the length of the X axis we use the two M3x10 screws of the x_end_idler while looking at the Z tops.

While you tighten the M3x10 screws keep an eye on the Z tops. The M3x10 screws will move the Z lead screws laterally (left or right), adjust them so that the Z lead screws are centred.

Note that the M3x10 screws will not move the lead screws forwards or backwards, only laterally.