zeroing z

To zero the z I would like to add a switch made from the tool bit and an
alligator clip and a piece of metal with a wire attached. The idea is to
be able to jog the z until the bit meets the metal sitting on top of the
work material. When the two meet they complete a circuit and stop
further jogging. At this point I can zero the z plus the width of the
metal.

Problem is what do I connect this z switch to? I am using deskcnc /w
driver. I need a NO input. Limits are NC - none on this board, homes are
NC and estop is NC on this board.

Is home the switch to use for this? Do I have to use a relay or
something to reverse the logic or am I missing something?

Thanks,
Phil