To measure the pump voltages, you want to have the pumps on (idling or system relay jumpered), and measure the voltage on both pump wires to a separate good ground. The +12volt (or so) wire should be close to battery voltage when idling. The ground wire should be close to 0volts. I wouldn't worry about a few 1/10ths of a volt drop. If you're seeing 1+ volt drops, then something isn't making a good connection.
What is the problem you're chasing? Have you checked that there's proper fuel pressure in the rail at startup?
[I think your gray wire at the tank is the fuel level sender.]