As I understand it...

1: Drop the torch till it touches metal.

2: Back off to pierce height

3: Establish an arc, wait for the pierce

4: Drop down to cut height and start moving X Y

5: After a short movement, measure the volts and save that as the benchmark.

For torch height I used 4mm studding with a 2:1 reduction pulley off a zippy DC motor. There is a chopper disk on the motor shaft for initial position and a digital voltage regulator to control the speed. I plan to measure the torch voltage many times a second then switch the motor speed/direction/brake accordingly. Not much of a plan but it's all I've got :D