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

Second image shows when the X axis is too short. In this case you need to tighten the two M3x10 screws.

Third image shows when X axis is too long. In this case you need to unscrew the two M3x10 screws.

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 even if the x_end_idler is not totally parallel to the frame.

Check the next step if you have difficulty to see if the x_end_idler is parallel to the frame.