Pull the vacuum line off of the FPR and plug the vacuum line. Start the car and set the fuel pressure to 39.3 psi. Anywhere from 38 to 42 is actually ok. Reconnect the vacuum line.
Since the FPR is tracking intake manifold pressure, you will see fuel pressure moving around as intake manifold pressure varies. If your fuel pressure gauge was referenced to intake manifold pressure instead of atmospheric pressure it would just sit at the target pressure all the time.
Anyhow, short answer is 38 to 42 with vacuum line on FPR disconnected.
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