Z axis offset with touch pad

I created a touch pad for my z-axis. Works great, and now I don’t have to guess how close to the surface the tool is. I was wondering, how do I set the z axis offset? My touch pad (a little spatula) is about .5 mm thick. So far, I’ve either adjusted the z axis down .8 mm (two clicks on the LCD), or just let it be off by .5 mm. I know it’s possible to set the offset, but how? I don’t usually have a computer hooked up to the machine, though I can for firmware changes. Can I set the offset in firmware?



Keep forgetting about the file size limit.

When you load the file into repetier you can manually move the z axis and reset the zero.
If you were 3d printing its built into the slicer. I checked to see if you code do this in gcode, you can but you would need to write a little script to z-home then reset the zero.