Z probe thickness is the P parameter within G38.2
It should look like this
G38.2 Z-20 F100 P0.5 (G38.2 probe and error if no contact-Z probe down Z 20mm- F feedrate 100 mm/min- P probe thickness 0.5mm)
But I would like to add that I too had a problem with the fluid dial probing menu adding 1-2 mm in the positive direction for Z0. I still haven’t dug into it in the code, but by running a probe.gcode macro I wrote, and an able to control the commands used, I haven’t had that problem anymore.
probe.gcode (282 Bytes)
On the subject of Z homing, the only way I’m aware of that you can home Z is if you have a limit switch installed. So “homing in positive direction” is moot because if you don’t have a Z limit switch, you can’t home Z. Zeroing Z is more common it seems.