Objective: What is a quick way to find where to hang a closet rod when making a hanging plant headboard?

Which of the following solutions is more sound in terms of naive physics reasoning?

pick from the following.
+ Mark a level horizontal line across your wall using your level and then mark all the studs you can find with your rare earth magnet
+ Mark a level horizontal line across your wall using your level and then mark all the studs you can find with your measuring cup
Mark a level horizontal line across your wall using your level and then mark all the studs you can find with your rare earth magnet