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8 PSI min - 80 PSI max
40 on idle up to 60 while on the move.
800 RPM should yield about 25-40 lbs. of oil pressure on a stock motor.
Need to know which pressure you refer to, fuel, radiator cap, tires, evap, ac, cylinder compression, transmission?
between 35/60 psi.
with your eyes
You need a vacuum gauge and it is reed in inches.
The number is the pounds per square inch
A manometer is a pressure gauge, "Manometer" reads extremely low pressures that are very close to atmospheric pressures, "pressure gauges" read much larger pressures.
It is necessary to read the density and the quality of the gauge. You should see the lower side.
Forget formula - put a gauge on it and read it.
Assuming this is a 4.3L engine, cycle the key off and on several times until the pressure peaks. The fuel pressure gauge should read 60 to 66 psi if all is well.