It's 39psi with the vacuum line off and the engine off.
Find your test connector on the driver side up by the fire wall. It's got one flat side and another side that is kind of bezelled. There are 4 connectors in a row on the flat side. The fuel pump test lead is leftmost one if the flat side is on the top. Ground that lead with your key in the on position and your fuel pump will run continuously. Check your pressure that way.
Don't set it for 30psi. Set it to 39psi which is what the 24# rating is determined by. You won't be wasting fuel. The only time you need to bump it up is when you are having fuel starvation problems at WOT.
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351W 89 Mustang GT Convertible