Hey Wayne,
the workpiece zero coordinate in your virtual 3D model has exactly to be at the same position on your real workpiece. If you define it at the bottom of your virtual 3D model, you got to probe or manually set zero coordinate at the bottom (machine bed) too.
If your Z height varies randomly when using the same g-code toolpath, you may have the Z plunge issue.
Also be sure that your touch plate dimensions are correctly set in the CNC controller.
- Support video: Onefinity Touch Probe Fine Tuning