These formulas produce rough approximations and shouldn't be used as a precise or accurate result. There is no way to actually cross reference the two other than doing actual cartridge side to side pressure testing.
There is also the problem of not everyone understands pressure CUP OR PSI and getting the numbers mixed up in your mind is a real risk.
[size=18px]USE AT YOUR OWN RISK[/size]
PSI TO CUP
(PSI + 17902)/1.516
CUP TO PSI
(remember your math ditty...My Dear Aunt Sally Multiply, Divide THEN Add and Subrtact)