When plasterboarding, you can have the long edge across the joists or parallel to the joists assuming that your joiust spacing allows this.
Which is the usual way to lay the plasterboard?
400mm centres should take a 1200mm wide board O.K. assuming the joists are reasonably parallel but there is a lot less wiggle room than if you are going across.
Reason I ask is that the board layout seems easier front to back, roughly 1.5 boards per 3 joists, with easier fitting around wall pillars etc. However a wandering joist could cause problems.