I assembled the x carriage as close to these instructions as I can but I have a few suggestions. You say to leave about 1mm between the motor and the pulley, when the y-stage is on rails the belt seems to ride higher than the pulley, it seems to bind when the y-stage gets close to the motor side of the x-axis. I moved the motor pulley higher up so it seems like the belt isn't too far from the motor pulley.
I also had to cut the M5 bolt to get clearance from the guide rod. Seems like it would be easier to just ship a shorter bolt with future kits so you make sure it has clearance. It was tough for me to pull the belt tight and tighten the idler pulley. I also think there needs to be mention of using a washer in the bottom of the idler pulley bolt so it holds into the wood better.