Aah, fuel tanks - we're actually just looking at them again for the X-plane version! The tanks are physically grouped into two areas in each wing and then 1 and 2 in the fuselage. Unfortunately, they are grouped differently in terms of which tanks feed which engine! As I said, I'm not sure of the code but you generally need to lower the fuel level evenly across all tanks to achieve a sensible outcome.
In terms of real world, display weight (so lightweight) was aimed to be around 135,000lb - remove all payload bar the crew and adjust the fuel evenly across the tanks until you hit the right area. Should probably be around 30 - 40 % per tank? Then, if trim is a problem, you can use the tray between the seats to transfer fuel to/from tanks 1 and 2.
Hope that's helpful,
Paul.