(fourthreethree)
As for the "quite advance reaching call" there may also be other reason for it to happen. Most autopilots/EFIS have a level mode (e.g. ALT HOLD) and few climb/desc. modes. Also, there would be a transition mode prior to level off. A good CRM would be to announce the transition mode in place, check altitude selection and baro setting so to ensure the aeroplane is properly set for level-off. Especially during descents into headwind, this transition mode can pop up when there is still quite some time to go. If a pilot would notify ATC at the very same time (reachin'..), the call would seem, well, premature. I was specifically told not to do so.
Which raises a question. It is my understanding (English not my mothertongue) that when instructed to, say "climb FL290, report reaching.", I do crawl up, level off, reach for the mike and scream "FluffyCloud590, FL290". That it was not actually required to report some time/height before HAVING reached the assigned level. Is this right at all?
(Warped Factor)
Now, is it right to think (and report) leaving a FL when in steady descent/climb out of the flight level, i.e. -/+ 300ft away from say 35000 feet? Beacuse while wrestling within 300 ft, you're still maintaining a level, right?
Although you may not maintaing your rating if on a check ride
. Also it seems to me that the paperwork would start piling up once one'd be off by more than 300. If in a lee wave, CAT or an engine out situation 300' would trigger "Unable to maintain" call from my side, if not earlier.
---
I thought that 200' is the quasi limit for transponder mode C crosscheck accuracy. Remember, the equipment replies with data pilot never gets to see! By virtue of appropriate training your baro is set so it correlates, but still you look only at the dial, not the aneroid/ADC. Where as the beeper sends out QNE all the time no matter what.