Not all X axis have the exact same length because of tolerances. This step will adjust the X end idler to your X axis length.

Adjust both M3x10 screws to have the x_end_idler parallel to the frame.

First image, top left: shows when the X axis is too short. In this case you need to tighten the two M3x10 screws.

First image, bottom left: shows when X axis is too long. In this case you need to unscrew the two M3x10 screws.

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.

It might happen that with excess tension on the M3x10 screws, the Z smooth rods will start to be off center on the Z tops. In this case release tension on the M3x10 screws even if the x_end_idler is not totally parallel to the frame.