It will take a couple of z axis calibrations to really sneak up on its accuracy and use something really accurate to check it. Digital calipers are good for this. Also check to make sure your wheels aren’t so loose that the machine flexes. My wheels just turn under hand pressure.
You’ll want to change $102 to adjust the Z. Here are the settings for GRBL.
Here is my response in this following thread as it’s a numerical ratio that produces the number you type in for $100, 101, or 102.
I first would make sure that the belts are tensioned, the wheels are tight, but not too tight, and that the pulleys are not loose on the motors. Then I’d put a straight bit in the router so you have a sharp edge to gauge the measurement. You need to keep doing the math until the movements are as accurate as you can get it. It’s a ratio of what you want to what you get. (20/19.875) x $159.70 = 1.006 x 159.70= $160.70 as the new number for the Y axis. These numbers could change after you tighten things up though…so it’s just an example.