My CNC has a Z height Probe only. It sits on TOP of my work piece and allows me to set my Z Height. MillMage appears to assume the Z Probe sits at the same height as the workpiece surface as I cannot fine a place to enter the Z Probes thickness!
So I have to use the 3 axis block rather than the tool length probe even though I can only use 1 axis! Sorry but that does not work for me.
The 3rd image in this posting may help.
https://forum.lightburnsoftware.com/t/probing-wizard-dialog/172885/2
Looks like you enter it in offset Z.