Interesting note. I looked at the CG on the Micheal2 mod for the 727-100. He has it at -8.5,0,0. And with that value the CG is just forward of center of the wing area, which is where the 707's CG is with the default (9,0,0). Of course they're two entirely different planes. But looking at RL EPR values for the 727, Micheal2 mod parallels the real word very closely, and does require flaps and trim for TO, whereas the 707, with the center of gravity in the same location gets airborne with no flaps or trim fully loaded (330k)
As mentioned, I guess the answer is to play around with it until you get something that seems real.  A real 707 pilot would be very helpful here. Maybe I'll call on Lou for this one.

Brian