Ducks vary more widely than chickens for example.  My bantam ducks typically hatch at 27-28 days.  My other ducks, except Runners, usually hatch around day 28-29.  My Runners a lot of times don't hatch until day 30.  I am not sure why there is such a wide variation since they are all developed form Mallards, but there is.  You just have to adjust accordingly.  Some large breeders and hatcheries (like Holderread's) stagger the days that they put eggs in the incubators so that they hatch at least closer to around the same time.  Maybe someone else that raises more Calls can tell you more definitively what they do.  Other than that, I would monitor air cell size and look for pip.