Hi Dave
Ah, OK, for a mill.
I use metal holding jigs a lot, and they are all made (machined up) with reference edges. I use the reference edges for alignment when reloading the jigs.
Frankly, I find sensitive commercial touch probes to be a bit of a pain, so I use the el-cheapo method of a ground 6.00 m carbide rod and a ground bit of shim between the rod and the reference edge. I find I can zero an axis to about 5 micrometres this way fairly quickly. That's for the X & Y axes.
For the Z axis, I often do just the same: use a shim between the cutter and a ref surface on the job. Again, 5 micrometres is normal.
One day I may set up a static ZTO at the sde of the mill table. But to use one I first need to use a mounted Z-probe to determine the difference in height between the top of the ZTO and the top ref surface on the job. I am working on that. Again, I find commercial 3D touch probes too expensive for this, too fragile, or too sloppy, so i will have to do something else, with a 1D probe.
Cheers
Roger