Captains,

In the real world, the CG% is listed on the load sheet that is given to the flight crew after the cargo and passenger assignments have been confirmed.  In simplified terms CG% is calculated using the dry weight of the aircraft, the weight of fuel and tank placement, and the weight and placement of cargo and passengers.  And just to confuse the issue more, you can have two different flights with the weights being identical but the placements of cargo and passengers slightly deferent and end up with a different CG%.

The only thing I can suggest for now is to use the default cruise CG% that is already in the FMC.  I believe it is 27% and gives you a trim setting of 5.75

I hope this is of some help.