1/2" at 20 feet isn't 1 part in 40. It's 1 part in 480 ;-)

Yeah, I really blew that one. I need more fingers to calculate with...

Lee Valley occasionally sells an inexpensive laser level (when they get
a shipment of them, for about $23 CDN) and they've tested it and found
it to be 1/4" in 100'.

Is it threaded for a tripod? Just curious.

Still, a decent theodolite, positioned at the exact center of the
building, could tell you a lot more about the geometry than a level could.