I think i see what your getting at. Whatever the delivery pressure (intermediate pressure) of your reg. is will determine the flow rate for any fixed orfice size. If you know the delivery pressure to the fixed orfice size, flow rate can be calculated in whichever system of measurement your using, metric or english.
I would think the supplier or manufacturer of the orfice could give you the performance data of that orfice for any given delivery pressure. Believe me when i tell you that calculating that on your own can get complicated. The equations to do so can rot your socks off, even if you have a technical background. I'm a Mechanical Engineer & it's been so long since i've done that stuff, i would probably do you more harm than good trying to explain it.
Calculating steady state flow of compressible fluids like air can be a real bugger unless your really familiar with the concepts. Check with the manufacturer, it's your best, safest bet.
Hopefully this helps, good luck!