You have the right idea, but the wiring diagram shows the ground off the sending unit. The guage gets positive and then I think measures voltage from it through the sending unit to ground. At the sending unit, the blue /black comes from the guage and the black/green from the low fuel warning.
The ground is common to both.
I could fax the diagram to you later today. It is all in the Haynes manual.
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe