SR71,
You need to get your hands on the
FMC supplementary data document for the Boeing Jet transport model xxx-xxx with whatever engines
This document contains detailed instructions on how the FMC calculates the Maximum Altitude. The document for the B777 has a chart showing
Thrust limited maximum altitude based upon a standard day with 100 fpm. There are also correction charts for Temperature and Residual Rate of Climb.
If you are unable to find the appropriate document for your aircraft, I will scan the B777 pages for you.
The Optimum Altitude based on a fixed speed provides the greatest nautical air miles per amount of fuel based upon the weight and temperature.
The Optimum Altitude based on an ECON speed is calculated for the complete route using the wind and cost index value.
Both of them are Optimum Altitudes
Mutt.