You don't need a $200 square to test the 90* accuracy of this or any other positioning/clamping square. A simple observational procedure of 4 tests determines their accuracy. Place one square inside the other and find the staggered gaps. Swap a square inside out and observe. Flip a square and repeat the above steps. Both of my positioning squares are true 90* inside and out. The only downside is that there is no slot in the center for adjustable CNC positioning, but I knew that from the picture.
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