I make a lovely little map of my downlights, with measurements taken from walls, and of and from adjacent joists, so that if the walls 'move' by being unexpectedly dry-lined then I can find the joists with my magnet and get my bearings off them. Works every time. It may take a while working out at first fix getting them all evenly spaced, and measuring and marking out at second, but the customer thinks its magic when I drill in exactly the right place and all the cables are there.
To quote a customer I went to see when someone had had a few 'false starts' finding their cables, and had drilled the holes too big for the lights so half of them popped through: "I've spent thousands on these effing ceilings, now look at them!"