Try entering different altitudes with one Cost Index. Note at what altitude the IAS to Mach cruise speed changes. Now repeat with a higher CI. Is the altitude for the IAS>Mach stay the same? Or does it change? Do it on a colder, and hotter, day at the cruise altitudes where the IAS>Mach occurs. Does it change or does the change over altitude remain the same? You should be able to figure some of the inputs into how the FMC computes the change.