Hello, I have a 1995 cadilac eldorado northstar engine, I would like to know what is the operating fuel pressure. Thank you.

With the ignition 'ON,' and engine 'OFF' (zero vacuum), system fuel pressure at the pressure test connection should be 41-47 psi. The residual pressure should not decay to below approx 20-25 psi, even overnight.
If the pressure regulator supplies fuel pressure which is too low or too high, a driveability condition will result.
That vehicle's fuel pressure regulators have been known to leak, causing extended cranking time before starting, especially once the engine is warm.
I hope this helps.