Usually we are using paper to calibrate the 4 corners leveling, some how it doesn’t that accurate as paper has 70~80 micron and occasionally the paper can be thicker due to residue from ur print, and it will scratch the paper. Here comes the feeler gauge the rescue!
100mm* 14 pcs ( 0.05~1.00)
Note: when measuring, try to keep it clean so it won’t affect the measuring result. As long as you felt there are minor dragging feeling, it means it hit the respective target.
For 3D printer, depends on filament, certain may need 0.2, or even lower 0.15, to 0.1 for filament that’s doesn’t stick well.