Need help with setting the right fuel pressure

Set fuel pressure to 40 PSI (manifold)

Mustangs come from the factory expecting 40 PSI DELTA fuel pressure (relative to the intake manifold). Any other pressure would need a custom tune.

How are you measuring/adjusting fuel pressure?

The regulator is intake manifold referenced (hence the vacuum line). That is how the FP remains at 40 PSI even under boost.

So if you are measuring FP with an external guage, disconnect the vacuum line. Set the pressure at 40 PSI. Reattach the vacuum reference. When the vacuum line is re-attached, you should see the FP drop at idle. Remember the intake manifold should be under a vacuum at idle.

Have a beer.

Just wondering. What kind of kit comes with an adjustable FP regulator? For a late model car, adjusting the FP is a good way to get into trouble for the un-aware.