It is unlikely to have to adjust the code, as thousands of printers are running this code - so unless yours has a some totally unique problem then you should be able to make it work.
From your console:
- G28 Z
- M851 z-20
- M500 (store settings - not 100% sure this is needed.)
(Be careful at this point it it entirely possible to crash into the bed.)
- G28 Z
- Move down in 0.1mm increments. (I use the motion controls on octoprint, not sure of the gcode).
Once it just grips the paper then you need to adjust the z offset as needed.
New z offset is -20+current Z height
I.e. if Z height = 9.3 then z offset is 10.7
M500 - store settings
G0 Z0 finger on power button just in case, check height when done, it should be in the ballpark, you might need to fine adjust with first few prints.
Remove all other z offsets from slicer.