I am using an older Hypertherm Max 20. It works well on 1/8", and will sever 5/16" if I work hard at it.

My question is - how do I determine what air pressure to use? I have tried boosting it up when severing beyond the real working range as it seems to blow the molten steel out of the groove better.

On my ESAB 550 it says to keep the air at 60-65 PSI and vary the amperage based on the thickness. I do about 20-25 amps for anything 1/8" and up and crank it to its max (27 amps) for 3/8" Seems to be very tolerant to what I set it to. I do notice a difference if the air compressor can't keep up. The cutting capability tapers off pretty quick.