I hadn't been able to perfectly square-up my 12" radial arm saw until I bought the Bosch DWM40L Miter Finder Digital Protractor/Angle Finder.  Using a large square, or my other protractor tools, the blade appeared to be at a perfect right angle to the fence, but cuts were ever so slighly off-square.  Placing one edge of this tool on the 12" blade and the other on the left side of saw fence, it read 89.7 degrees.  Placing it on the blade and the right side of the fence, it read 90.3 degrees -- the perfect complement of the first reading.  After a good bit of adjusting on the saw, I was finally able to get a 90.0 reading on both sides.  Now, the radial arm saw cuts accurate right angles.  I used this tool to "perfectly" set right angles on several tools, and have used it sucessfully in construction projects to precisely match angles.