Just taking a swing at this...but it sounds like the goal of 'this flight' is a constant MACH trip..and if that is the case, then a hypotheticaly planned trip at .78 mach at intiatally FL280(or something) would be mean that as your MACH increased to say .80...you might request a higher alt, to put the plane back at .78...and somewhere in this process you might end up at a certain alt, and then at this certain alt to keep .78 for the remainder of the trip, as your weight decreases, you reduce thrust....
Well I guess that's one way to plan a trip....