Reply by jdh122

If you go in person to buy your square, you can check it in the store using a pencil and the flat edge of a shelf. Just keep trying different ones until you get one that works. Also, combination squares can be trued up with a piece of sandpaper: http://www.instructables.com/id/How-to-square-a-combination-square/
I have and like one of the PEC combination squares but if you’re willing to fiddle you can tune up a cheaper one.

-- Jeremy, in the Acadian forests