With the ceiling up, I can mark up and drill the holes. It took quite a while to get these right, and you can see one of my crossed-out mistakes in the picture. But in the end I think they're OK.
Since it was the weekend, and the tile cement would be dry by the time the guy came on Monday to finish the ceiling, I thought I'd put down the first row of floor tiles. Woooah! Not level. That's going to have to be fixed.
They fixed it with a layer of self-levelling mortar, something I didn't know about until now. It's quite liquid and flows to a near-level surface without much intervention needed. Takes a couple of days to set, though, so it was the last thing to be done after the ceiling was finished.