You are right with the frame and there are reasonably cheap lasers, then it becomes exact.
It is also correct with threaded rod, take 6 mm then (I don't know if there are thinner ones), that will do fine.
Then it can be adjusted exactly by the mm, the attachment in the ceiling, maybe the plumber's Z-iron with washers can be a good mount in the ceiling and with some string to follow for the mounts.
It will probably work, and the nail will be straight
It is also correct with threaded rod, take 6 mm then (I don't know if there are thinner ones), that will do fine.
Then it can be adjusted exactly by the mm, the attachment in the ceiling, maybe the plumber's Z-iron with washers can be a good mount in the ceiling and with some string to follow for the mounts.
It will probably work, and the nail will be straight
Thanks, then it will be like that, good to have a sounding board.