The 'required' flow rate comes from looking at the various possible cases that could overpressure the vessel. The various cases depends on the process system you have and the various design conditions. If you haven't already done so, get a copy of API 520 and API 521 and go through them. API 2000 is the equivalent document for tanks.
The 'actual' flow rate comes from the selected PSV. When you identify the relieving case, you use the flow you have deteremined along with the fluid's properties (P, T, MW for gases) to determine the relief area you require. The formulas are in API 520 and PSV vendor catalogues. PSVs come in specific sizes so you pick the next larger orifice. You can then use the actual orifice to back calculate the PSV's capacity. This flow rate is used to size the inlet piping so the pressure drop is less than 3% of set pressure. It's also used to size the outlet piping.
Sizing the outlet piping is a matter how much back pressure you can allow. The pressure at the outlet of the PSV has to be greater than the destination pressure (atmosphere, flare system, etc) so it will flow and you calculate that from a pressure drop calculation.
If you have a conventional PSV, the built-up back pressure (due to the DP needed to get your flow out to the final destination, whatever that is) is limited to 10% of setpressure. If you exceed that, you either need a larger discharge pipe, balanced bellows or pilot operated PSV. Balanced bellows PSV will accept up to 50% back pressure though there is a reduction in capacity above about 35% (depending on the relief case).
If you haven't read API 520 and API 521, you really need to do that as a first step.
