The flow gauge reads in CFH (cubic feet per hour), not in psi. The standard flow rate for MIG welding is 20CFH but you may be able to get by with less depending on you conditions. The gauge will probably read different with the gas flowing vs not flowing so you should check it while the gas is flowing. Too high and too low will both cause problems. If it is too low you will get weld porosity, too high can also give you porosity from turbulence allowing entry of air.