toolsensor below Machine Surface

Hi
i am trying to get met tool sensor to work in 2.8+Gmocappy
the sensor is mounted below the surface of the machine table

in the settings tab I can specify the X and Y position.
The Z-value only a positive number can be entered. Manually changing the   xx.pref file doesn't make a difference.

In the past (v2.7) my setup has worked this way, I only can't seem to set it up now.

any suggestions?

I have moved this to the Gmoccapy section in the hope that Norbert will see it.