mig regulator PSI questions

I think I run about 15 with straight CO2 which is what I use for "all" my mild steel

But to do it right, you don't use pressure, you use FLOW. You can actually buy an inexpensive flow meter which you point the gun "up," disable the wire feed, and sit the flow meter on top of the gun. As with most simple flowmeters, they are a tube with a ball inside.