Compressed gas flow rate calculation

Folks, I'm new to this thread. I'm a mechanical engineer, but I haven't done much fluids work since I graduated, 20 years ago. I have a question about calculating gas flow rate, so any help would be greatly appreciated. I have a design that I'm working on that will use a compressed gas cylinder to push a plunger that pushes a fluid on the other side of the plunger out of a critical orifice. I need a very low constant pressure (about 1 psi), the cylinder manufacturers are telling me that I need a flow rate. I've told them that the gas will fill a chamber with a piston and the total volume of that chamber is 3 mL. This filling will occur over a period of 16 hours, so that flow rate is 0.188 mL/hr if it were a liquid. I'm not sure how if that's the flow rate they are looking for (and neither do they) since it's a compressed gas. Any ideas on a governing equation(s) that I can use.