I'm attempting to choose a pressure regulator(for use with an inert gas) for a personal project I am working on. I know that the inlet pressure will be 2000 psi and outlet pressure of 300 psi. I also need the mass flow rate to be approximately 1 g/s. I've done some calculations with the perfect gas law and got a volumetric flow rate of 415.943 SCCM based on the inlet pressure. The Cv's I've calculated thus far does not seem correct as they are very small numbers. I'm wondering how to properly choose a pressure regulator that can fit these parameters and how to calculate the Cv properly. Any help is great!
Don't let the small $C_v$ values throw you off. Many pressure regulators for research applications work well in the well below 1E-5, 1E-6, and even much lower. You may already have a good calculator, but this EngineeringToolBox page has a gas $C_v$ calculator that is easy to use.
But I also question if you have resolved what will control the mass flow. Can you clarify whether you primary objective is flow control? It could be you need a pressure reducing regulator to protect the mass flow device, but, as suggested above, a PRR will not control the flow for you.