My friend, there is no difficult thing in sizing PSV as long as all data required is already provided. The all we need is just inputting the data in the formula or in the spreadsheet :D , then select the appropriate standard orifice size ( refer to API STD 526)
The following pictures, show the formula in API STD 520
For non critical condition, the outlet pressure has impact in sizing orifice area but for critical condition is not. In critical condition, the outlet pressure is lower than the critical flow pressure therefore there will be no impact to the sizing.
For you who looking for the sizing example, click the following link.
1. Gas/Vapor Critical and an example from Leser
2. Gas Subcritical and an example from Leser
3. Steam and an example from Leser
4. Liquid, an example from Leser
Thank you for your attention.