I attempt to explain how to level the bed of an Ender 3 using a dial indicator from Harbor Freight.
I used a 5 thou feeler gauge. The exact feeler gauge thickness doesn’t really matter because it needs to be appropriate for your printer. You might need more or less to get good adhesion. I’ve since switched back to the regular paper method.
I used this feeler gauge: https://www.mcmaster.com/2334A8