If you can't get any further, why not experiment? With the FMC's on the 757 and 767 in my airline (not Pegasus), there are a couple of differences. For instance, on the 757 the max altitude will change depending on the climb derate selected (although it will not re-calculate until you change something else in the FMC). The 767 does not. Max and Optimum alt will also change depending on selected cruise mach.
Cost index speeds will change depending on winds entered - slow in a tailwind and fast in a headwind. Its worth playing a bit, you will discover all sorts of odd things. Having said that, I wish they gave us better books to describe the operation properly.