At my work an admin kept insisting that the TV we had jsut hung was crooked. When we finally sent a guy up to check it turned out her iPhone said it was 2 degrees out of level. 3 normal levels said it was level. Guess which we believed?
I would never trust the sensors of a phone for something like adjusting level. They're simply not precise enough.
Also, an application i recently discovered for Windows/Mac Os/Linux that is awesome for home design is Sweet Home 3D. Is open source, and has enough features to design your house from top to bottom. And is relatively easy enough to use (at least easier than AutoCad)