Determining Back Pressure for Lube Oil System

[chemecar]

hello engineers, i am currently working on sizing a relief valve to go near the pump discharge for a lube oil system. the only thing left to calculate is back pressure. PLEASE VIEW ATTACHED FILE. i was hoping for an explanation as to how the 22.5 back pressure is calculated. the oil resevoir is atmospheric and the normal operating pressure is 22.5. is it a coincidence that the back pressure is the normal operating pressure. this info is needed to size an rv.

 

[chemecar]

sorry guys, i'll put up attachments later, i just want to know if the normal op pressure can be set as the Back Pressure.