Hi Rat 5,
Inserting the correct number will raise the Max Alt displayed and that is the real aerodynamic Max Alt.
You should have the correct number inserted into the FMC from load and balance calculation.
On a previous type we were permitted to reduce the buffet margin from 1.3 (max 30 degs bank) to 1.2 (Max 15 degs bank) provided there was no turbulence forecast ahead. By using fuel distribution graphs, we could insert a new cruise C of G figure which then displayed a new MAX FL.