My Hypertherm powermax 45 xp plasma cutter sets its own air psi electronically. You want 100-125 psi air supply to regulator from air compressor, then set air psi at plasma torch to 70psi on most machine torch applications.
Don’t need a flow rate gauge, I have 3 hp ,60-gallon air compressor with 3/4" air line from my compressor to my filter system, no air flow or air supply problem.