Laser levels are very cheap now a days. I have two. the first one was more expensive than the second because of tech and mass production. You can get a 4 foot laser level for under $25. The swivel base for a steady broadcast of laser point is the expensive part, but you can easily build one with a simple lazy susan guizmo and a tri pod.
All you want to do is find the center of the room, locate the tripod there, then be able to set the level on that pod and gently move it all around the room pivioting on the susan. No big deal at all.