Make a jig and use some software for repeatability
Some very clever people have made this task very accurate and quick.
I first read about a Z axis zeroing routine for Mach3 on the CNCzone, but never got around to trying it. However, that original post has blossomed into the most useful addition I have made to my machine. You can now easily zero all three axis, as well as locate these zero's on the appropriate corner of your workpiece. Full step-by-step directions and explanations are given in this post on the zone.
I have made one slight modification to my jig. I used the version JoeCNC created as discussed by erniebro in the post, but instead of a pipe, I used a 1/2 inch copper end cap. This way, after I find X and Y, I can probe immediately without changing the setup to find Z. I also modified the Visual Basic to directly zero my X and Y DROs, vs. re-set them myself.
Here is my jig in use on the Romaxx,
the bottom side view of the jig,
and the VCarvePro file for the jig.
Results from running the routines:
X-Alignment
Y-Alignment
Z-Alignment
Mach3 screenshot showing the control:
