I always initially mark out from centres and see what that looks like. After marking the walls and floor and taking into account the bath, shower, window sills etc. Like Acaciaguy says, it can take an hour just getting it right.
Man, if a customer walked into a bathroom or kitchen that i was marking out they would wonder what all the scribbling on their walls and floor were, and coupled with the pencil sharpenings on the floor they would probably wonder wtf!! lol
my walls look like they have been assaulted by a box of sharpies 😀