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Short answer:

My Powermax is paired with a THCAD-10.  I changed it to 20:1.  It’s served me well with no issues over the years.

Long answer:

forum.linuxcnc.org/plasmac/39999-thcad-1...powermax45-xp#181612

It depends on your machine amperage. No scaling resistors are rwquired. Most people use 20:1 which gives a 200 volt full scale, 125 amps might be better at 30:1. (300 volts)
The thing is that the THCAD is rated for overvoltage of 500 volts. The voltage will only go over 200 volts on piercing when we do not care about the voltage then. During cutting voltage is between 100-170 volts. Keeping the max voltage range low means the readings are more accurate as more of the ADC's range is used. 50:1 is popular with a lot of non-linuxcnc THC's  (probably with a 0-5 volt ADC). That would be better for a THCAD5.

50:1 worked well you can use the THCAD10 without any modification, 
Some good practices are:
-Place THCAD10 as close as possible to the Plasma power supply.
-Use a shield cable and connect the shield to GND on one side only.
-Use a ferrite bead on the control side to reduce induced noise