HEALY,
To follow on from Keith.Williams'. excellent answer -
There are a multitude of Optimum Altitudes for the Jet aircraft, it all depends upon the chosen Speed Schedule. Speed Schedules vary considerably, for example -
(1) Minimum Fuel (Maximum Range Cruise),
(2) Minimum Time (High Speed Cruise),
(3) Minimum Cost (Best Time / Fuel compromise), variable on any given flight.
(Long Range Cruise, i.e. 99% of Optimum Maximum Range Cruise, is a simplified version of Minimum Cost operations).
Each of the above will have a different Optimum Altitude, and in the case of Minimum Cost, will have considerable variation depending upon the Cost Index for a specific flight. Periodic checks of the FMC in cruise, examining all of the above options, will indicate differing Optimum Altitudes for each case.
Considering still air conditions, the Optimum Altitude for each of the above speed schedules, excepting Minimum Time, will steadily increase as the aircraft becomes lighter. Minimum Time operations, i.e. Maximum TAS, will be at a fixed Pressure Height, that is the P.H. at which the scheduled High Speed CAS equals the High Speed Mach Number.
Regards,
Old Smokey