Its not setup in mach , it is setup in your post processor. In sheetcam my post processor has a place for me to put my offset which is .26
This is the code it then produces for every pierce.
N0110 G28.1 Z0.12
N0120 G92 Z0.0
N0130 G00 Z0.2600
N0140 G92 Z0.0
N0150 G00 Z0.1300
It makes the z go down until the switch is tripped. It sets the z axis dro to 0. It raises the z the offset number (.26 in my case). It again sets the z axis to 0. Now the top of the metal is at 0 and the the tip of the torch is at 0. It raises the z axis to the pierce height. ( I use .13 in this case) and starts cutting.
Hope this helps.