THC works by sampling and monitoring the voltage between to the torch (-) and the work(+).
This sample is essentially taken at the output connectors inside the plasma machine. Variations in cutting height (warped material, etc.) varies the current load on the output, this in turn causes minute variations in the voltage. As the gap between tip and work increases, the electrical load decreases and the voltage rises, the THC circuitry and firmware senses this and signals the Z-axis to lower the torch in an effort to regain the norm. The converse happens when the tip to work gap lessens.
Sorry, but this was a long-winded way of saying the voltage can be sampled at any convenient point, usually across the output connector or at an OEM supplied connector at the back of the machine. I’m not familiar with the Miller 375 X-Treme, but the theory holds true. Hope this helps.