I thought about using my laser. The problem is the tri-pod doesn't come anywhere near close enough for high work like that. The drywall is also in 4x8 sheets but it just makes up the wall. I have to suspend the grid work from the ceiling joists after first installing the framing to the wall all even with the lowest point I'll need to avoid overhead obstructions. It sounds like a fantastic headache coming...This is for a suspended ceiling?
Yup, it's a mission. Flat, level and plumb always are.
I used a lazer level for my roof beams. But, having the beams open underneath and ply on top which has some flexibility made life easier. I'm not sure exactly what your drywall sheets are. In the UK plaster board is used and it comes in 8' x 4' sheets and they don't flex much and they weigh a ton.