This weighs heavily upon my mind... Laser transit and a precision square. Level is irrelevant, square is key. A lathe will work upside-down as long as it's square. Imagine trying to set a lathe up on a ship, how would you make sure it's flat? Measure from one fixed point in the room to six points on the lathe. Spirit levels are for masons and carpenters, machinists measure from a known point.