You don't seem to be telling us why you want to know the car fuel pressure so it's hard to give you good advice but it also seems you have some misgivings about how a car actually works.
When a car starts after sitting overnight it is running in open loop mode because the oxygen sensor needs to be hot to give a good reading. The oxygen sensor is what closes the loop in the control system because it gives direct feedback on whether the correct amount of fuel has been added.
In open loop the car uses a multi dimensional look up table. The look up table accounts for RPM, intake air temperature, intake air flow, engine coolant temperature, load, barometric pressure, just to name a few. This look up table then gives the car the necessary amount of fuel to add to each cylinder. This look up table is painstakingly generated by the manufacture in a special engine dynamometer. This dynamometer can literally control every aspect of engine operation; load, coolant temperature, engine temperature, fuel pressure, etc... The dynamometer can also test the exhaust stream for the standard 5 gasses related to emissions. With this dynamometer engineers meticulously analyze every point in the look up table compensating it for everything they can think of, like the range of allowable fuel pressure. The values in the look up table are generally mid to worst case scenarios. For example if the fuel pressure is at the low end of the allowable scale.
When the oxygen sensor gets hot and starts giving a reading, that reading is used to verify that the correct amount of fuel is added and compensate for when it's not. This is reflected in the fuel trim numbers. When the sensor sees a rich mixture when compared to the open loop look up table it will take fuel away resulting in negative fuel trim.
Unless the car already has a fuel pressure sensor, the fuel pressure is compensated directly into the look up table and does not require calculation.
Theoretically I repeat Theoretically by comparing the delivered amount of fuel in closed loop to the open loop look up table you can infer how far away from the optimal fuel pressure you are. This depends on too many variables to truly be accurate. You need to know what that pressure that the engineers compensated the table for. The engine has to be in perfect running order. The injectors can't be even 1% plugged or your calculation would be off.
Also notice how i didn't mention oil pressure in there at all or whatsoever.