If you set the distance between the two clamps exactly as written in the manual, chances are that the movement is only OK in the center and difficult near the end of the axis. I think most users enlarged the distance between the clamps by a few tenth of a mm so that the axis move smoothly along the full axis.
This is really, really important for a proper functioning printer in the end!
I removed the print head completely and first figured out the best distance between the clamps after which I put everything back together with those newly found distances.
Then, I aligned the x and y axis to the steel rods, not to the plastic housing.
When I got it about right and was able to start some prints, I immediately printed these guides to fine tune the alignment: