Using spoilboard to set Z=0

Hi,

I am wanting to use the spoilboard as my reference for Z=0. There are a number of ways to do this of course, from setting the Z-probe on the spoilboard to manually jogging and using a piece of paper to determine when the bit touches the surface.

But what then? How do I set the Z to zero without affecting the X and Y zero settings? If I could set Z=0 independently, that would be perfect. Then I could jog to the workpiece origin, and set the X and Y zero. But I just don't see a way to do this.

Is there anyone who uses the spoilboard to set Z=0? If so, how do you do this?