Thanks Don. Information obtained from "baseops" is very comprehensive but also too complex for me.
Since you have offered to answer, I would like to take this opportunity and ask the question again with two examples:
1. - Optimum flight level for a distance of 100 nm
2. - Optimum flight level for a distance of 1500 nm

Assume that the load is the same and that the course is indifferent. The flight is IFR. Levels that you would use and that basis are you to determine them?

Thank you very much.

Antonio