Quite simply - the actual level at which the changeover occurs is dependent upon the ambient temperature and pressure. You cannot say it always occurs at FL280 - it'll be around there but will vary on the conditions.
Just fly the thing at the scheduled climb IAS until the Mach meter reaches the scheduled climb Mach - - usually it will be the same as the cruise Mach No. At least it was in my old jet!
Rgds
1106