Use this little spacer to level your 3018 CNC bed
Process to calibrate

First connect a multimeter with alligator clips. Put it on continuity
Then clip one to the bed, one to a large bit (must touch the bed, but be capable of going at least 2mm up)
Then put the bit in the center and lower until the multimeter shows continuity
Then go slowly up (by 0.1mm increments) until there is no continuity
Thats your Zero
Then go up by 2mm and move to any corner
Then repeat the process of going down until there is continuity, and take note of the last value with continuity when going up
Repeat until you take the 4 corners
Mine look like

+0.2 -0.6
0
-0.2 -0.5

So import the model in the slicer, and resize its z (ENSURE TO DISABLE "UNIFORM SCALE" in the slicer) to be the correct size
then release the screws a little bit and slide the spacer under
AAAAAnd repeat al the process until you are satisfied