Precisely level your 3D printer bed using a cheap and readily available tire tread depth gauge.
Includes frame to attach the gauge to Sapphire Pro and Plus, as well as Creality the printers with a single cooling fan.
Level one corner using the paper method, zero the probe then move the print head to the other 3 corners and adjust until the probe reads 0. Then zero the original corner and re-check the other 3 corners.
Fine tune by printing a single (or double layer) square over one corner, and adjust until you have good bed contact (squish). Re-attach the probe and zero it over that corner, then transfer the zero to the other 3 corners. You should now have a perfectly level bed.
Note: The gauge can be modified by removing a bow shaped metal spring from the inside. That allows the probe to follow the contour of the bed as the print head is jogged around. Otherwise you need to pull up the probe before moving the print head, and push it down when you want to take a reading.
If you have a BL touch, or other probe, you should also keep your bed levelled as to minimise the adjustment needed by the mesh levelling. That adjustment is gradually removed over successive layers, so any levelling error will be reflected in the geometry of your part.