Hi I am putting my CNC together again i built some years ago. It runs on two linear rails on the x-axis. They are about 4 meters long. I know already how to bring them parallel in y axis. With having a straight master rail and then align the slave with a dial indicator. But now I wonder how i could align them in z also. I don't have a flat ground plane i could adjust them to.

What I have is a straight edge, a square and two dial indicator and two semi good levels. Any recommendations?