When setting out for a plain wall, you should find the high spot of the wall. The high spot will be a brick or concrete or something that stands out from the rest of the wall. You have to find the high spot and work from there because otherwise the board will not be flat with the rest of the wall and that will lead to not having a good finish.
The high spot should then be moved up to the top and marked and the same to the bottom.
From the mark on the ceiling, add 10mm for the thickness of the adhesive plus the thickness of the plasterboard.
The new measurement should then be taken to each end of the ceiling and remarked so that you know where the plasterboard will be fixed and everything will be the same thickness.