I'm not sure if this would work, but you could change the fuel quantity in the default flight for FSX/P3D, save it, then see if that affects the CS 757's fuel quantity.

Edit. I just tried this, and it worked for me in P3Dv4, so I can't see why it wouldn't work in P3Dv5, or FSX/SE. I changed it to 50% in all of the tanks in the default flight (F-22 Raptor) and the 757 had 50% fuel in all of it's tanks (left, right, and center) too, but I don't think you want 50% in the center tanks as well.