This is the holder for the digital machinist Dial Indicator from Harbor Freight ( https://www.harborfreight.com/saemetric-digital-indicator-63613.html ).

Simply connect the holder to the extruder head using some rubber bands. Insert the dial and secure with again some rubber bands. Lower the bed to its lowest point. Lift the print head 5 to 10 mm off the bed. Set the dial to zero. Now measure from side to side using move X (0 to 220mm). Level the bed using the screws and secure. Measure again because tightening the lock screws can unlevel the bed by as much as 0.2mm. Recommend using NAK 3D Y-axis leveling method https://www.youtube.com/watch?v=3c2PW5GNZrE&t=308s

Yes, this is overkill for a 3D printer, but I was having issues with bed leveling. My bed is 0.4mm lower at the homing end than the non-homing side. Following Crealites bed leveling instructions was not working for me. Though NAK 3D’s method helped but still was having issues. Fortunately I had the dial indicator laying around and printed the holder for it. Bed is perfectly level, and I will not need to touch it till I have to replace the belt.

Originally printed it using resin (FDM printer took sick leave), but prints easily on the CR-30 after leveling.