This has vexed me as well.  I haven't been able to find a reference table listing appropariate CG ranges.   Nonetheless, others in this forum have said to go ahead and insert the default value listed in the FMC (27% I think).  I do that but have found that the aircraft can sometimes nose up too easily slightly before Vr.   I generally add a few clicks of nose down trim from my joystick to compensate and don't have any problems.  My apologies to CS for this clumsy work-around in an otherwise superb sim.  

Chris