That "12 miles away" part is the thing that trips people up. The earth curves about 8 inches per mile squared, so over 12 miles the drop should be roughly 96 feet. But that math assumes you're measuring from the surface straight down at the far point. If the laser is sitting a few feet off the ground and you're looking at a target that's also a few feet up, you're slicing through the curve at an angle, not dropping straight down from it. You'd also need to account for refraction, since air bends light over water and can make things look flatter than they are, especially on a warm day at Tahoe.

Drop cloths kinda saved my butt too but not for the reason you're saying. The trick isn't the cloth itself, it's that you gotta tape the edges down with painter's tape or else soot just slides under and you're still mopping. Did a gut job last spring and skipped the tape on the first room, total waste of time. Once I taped everything it was night and day. So yeah the old guy thing works, but only if you do the boring part everyone skips.