I think 747diesel dude has pretty much hit the nail on the head. I've looked up the cruise performance and they top off at what is CRZ setting on the N1 tat computer. Fatigue life.
We are pretty much always close to max take off from HKG and when in initial cruise ,I like to leave it in CLB for the extra margin. Ive had, on many occasion, with CRZ selected in turbulence, where the speed as dropped and won't come back up cause of the CRZ n1 limit. Back to CLB for a while.Under 300 tonnes I think CRZ has enough to maintain speed even in mod turbulence.
Our books have been amended and the fratts operations has been taken out except to say to put it in CRZ when reaching cruise speed. In the old book, it said : when reach cruise speed or slightly above, select speed until stabilize to required mach number (about 10 minutes, then select mach) If anyone has seen this part in their books, I'd like to know if I saw right.