General setting is twice the cut height. One way it is figured out is a floating touch head. When the head is all the way down the switch is engaged when it floats up x amount. It then pulls the head up that x amount when it touches off.
Now the tip of the torch is on top of the metal. The program now raises the head to twice the cut height.
Now you have to have a program that creates the touch off code or manually put it in to your gcode for every pierce.
I hope this makes since.