I agree with Denti's point that it is hardly worth the bother. The usual way you check the OPT/MAX is from the FMC. But if you are in ECON and then enter the exact same mach number you will get quite different OPT/MAX numbers. Enter a higher or lower MACH and you can massage the margin whichever way you want, which may or may not be a good thing, depending on whether you know what you are doing.