Hi SR71,
I daily put the correct CG in the PERF INIT page to obtain a higher Max Cruise Altitude. Depending on the FMC software version installed, the CG in the FMC is default set to 5 or 15 (or perhaps other values).
Changing the default 5 value to the actual value (for example 18), will increase your max ceiling by a few hundred feet, just what you need when ATC asks you "are you able to climb Level ...". Changing from 15 to 18 won't give you much.
You can enter the Takeoff CG for a start, but you must update this CG during flight. Use the fuel curve printed on the manual loadsheet to verify how CG moves when you burn fuel (this is not a straight line from TOW to ZFW in the W&B enveloppe).
Hope this makes sense
Pat