I bought a borg framing square after grabbing several and checking one against another on as flat a surface as I could find. I figured if two agreed, they'd have to be out of square the same amount and in opposite directions. That seemed pretty unlikely. I bolted 1/4" thick wood pieces on either side of the shorter blade with the wood extending slightly beyond the metal. I then scraped the wood until it was flush with the metal. I checked the result using the 'draw a line, flip the square and draw a parallel line' test. It seems close enough for my purposes.