We would get the joiners to plane some thin timber into a feathered edge and screw to the wall where it meets ceiling angle, levelling the feather edge and margin it from floor. Would then scrim and bond entire ceiling, getting it tight to the feather edge but just a few mm back to allow for skim coat, trowel up and leave skimmed and edge on for a day, unscrew feather edge and viola!!! This has worked a treat for us when we have needed to do the same. Our preferred way is to put a floating stop bead across wall, same principle as feather edge, and bond ceiling with a slight round onto bead, this separates wall from ceiling and eliviates that crappy angle that never seems to be straight, but your client may not go for that, in my experience they are usually fine with it as the bead is straight and you also have the break between ceiling and wall so they can paint two different colours if they wish.