As Matt said about 20 CF to start but just like a regulator every rig will have different requirements. Use the same method as adjusting a regulator, while welding on a piece of scrap slowly tun down the gas flow until porosity appears then turn it back up a couple of CF. You may find you need more than 20 CF and if so make sure the nozzle is sealed good at the back and not siphoning air into the flow ( a common problem that can cause a mysterious porosity problem) but you may also find you don't need quite that much flow. In that case you can save a lot of your gas over a few hours of welding since even a few CF of wasted gas adds up in a hurry, but again like Matt already said about 20 CF is most likely going to be the magic number.
BTW, you are much better off with that flow meter than a regulator anyway so you are upgrading your equipment not just replacing a broken part!