You want the needle to stay in the green at all times. Here is a colorize fuel pressure gauge to give you a clue of the fuel pressure span you want. It should never fall below 10 PSI at all.
Normal fuel pressure should float about 14-15 PSI running down the pavement. This is for your Dodge Cummins Turbo diesel truck with a Bosch VP44 injection pump.
Here is a copy of the fuel system specifications from the Dodge Factory Service Manual. Fuel Pressure Specification For Bosch VP44 Injection Pumps
