According to R Barr :

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.

I think so. If not, you could always duct tape it to something with a hole
in it, and bolt it to a tripod. With some calibration checking, it'd work.

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.

A decent theodolite would do the trick of course. But one would usually not
be cost effective for just one or two jobs. Whereas, a laser level has somewhat
more flexibility/use, vastly cheaper, and with a little ingenuity/perserverance
would probably do about as well.

I figure for less than the cost of renting a theodolite, you get something
to keep and use for other things too...

Not something a pro would want to screw around with, but...
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.