PandaPi and Plotter

[patofoto]

Unsure where to post my issue. Have my PandaPi running the latest version of Marlin with OctoPrint. I am using my PandaPi for a custom made plotter. I want Marlin as I want to use manual mesh bed leveling to create a mesh to try to keep the pressure of the pen or brush as even as possible against the paper. Everything is working as it should except when creating the mesh. I am using G29 commands to create my mesh. The first command is G29 S1 preceded by G29 S2 for each of the following mesh points. My problem is that the probing sequence starts at X0 Y0 Z0 which is at the position where the limit switches get triggered. X and Y positions are no problem since, from this first probing position, I need to move on the positive directions to probe the rest of the points. Z is an issue. I need to be able to move in the negative direction to take account for unevenness in the papar surface. Since the head is hitting the end stop, I cannot physically move. How can I trigger an end stop but have zero Z have an offset?. Or how can I have the probing process start at a positive offset?. Have tried tweaking Marlin firmare for hours and cannot get it to do what I want. In my head it should be something simple, and probably is, but I am burned out and cannot think clearly anymore. Any help or printing in the right direction would be soooo appreciated. If this is not the right place, please let me know.