This relates to those printers which have a stepper on each side of the printer to control the position of the Z axis.

Before you can successfully make fine adjustments your extruder's Z= 0 position you have to get both sides of the slide of the X axis roughly the same distance from the heat bed, so that the value for Z is roughly the same on the left and right hand sides of the printer. You can get make a coarse adjustment of the X axis by lowering it onto two things that are the same thickness (like pieces of dressed wood, or two similar rulers placed on their edges) by turning the Z axis threaded rods . I used some scrap wood that was in my workshop to make this coarse adjustment. It does not matter how thick the things are, as long as both are the same.

Here is a picture of my how I made this coarse adjustment using the scrap wood:

Coarse adjustment.jpg

If you want to get really anal, you can slip a feeler gauge between the wood and the slide so you know that you haven't squashed into the wood when you wound down the X axis.

Old Man Emu