Sorry, yes, the fuel pump must be running. To do this without running the engine, you ground one of the leads in the self-test connector with the key on, engine off. It's also possible to flick the key on, take a reading, adjust, flick the key on, etc., but it's best to just ground the lead.
If you look at the self-test connector there is one flat side and a beveled side. With the beveled side pointing down, the lead you want to ground is on the left side of the connector, or, in other words, it's the one that is closest to the group of four in the middle of the connector.
Fuel pressure with the line connected when the engine is running depends on the engine. It should be set at 38-39 when the engine is off and the vacuum line disconnected.
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351W 89 Mustang GT Convertible