There is no standard number for that on a modified engine. Every different cam grind will idle with a different amount of vacuum. If you install a vacuum gauge you'd know exactly what you should see. If you idle @ 16" of vacuum that is -7.86psi ... roughly 8psi of fuel pressure increase you should see. Figure every 2" of vacuum to be 1psi of fuel pressure rise you should see when the line is removed.
