What I am talking about has nothing to do with the Xponder.
The aircraft I fly has two modes for cruising, one does a very good job on maintaining the FL and Mach Number but it needs alot of throttle adjustment
the other mode allows the aircraft to cruise at FL +/- 100 ft requiring less throttle movement hence less fuel burn.
What I will like to know is how accurate has to be the level keeping?
P.S. Does PPRUNE has a Regulator Forum?