Just to add to what everyone else has said...
API RP520 (Part I) has a lot of good information as well for PRV's.
I would also note that the mass flow rate will differ depending on whether the flow is critical or sub-critical. Q_Goest's equation is for critical flow, however I would also consider the compressibility factor (since it is gas relief) and even an additional K_b capacity correction factor for back pressure (this is in addition to the discharge coefficient).
Typically I would check to see if the flow is critical or sub-critical first, then use the appropriate equation based on that. If the pressure downstream of the nozzle is less than or equal to the critical flow pressure, then critical flow will occur. If the downstream pressure exceeds the critical flow pressure, then sub-critical flow will occur.
Just my 2 cents...