When I first received this product, I was excited with its potential.  First I tested it on several bonafide squares, but the Bosch consistently measures off by .3 degrees.  Not huge, but enough to be concerned about when buying a precision tool.  I searched the instructions, only to learn there seems to be no method an end user can set it to a 90 deg benchmark.  In my opinion, this is a shortcoming.  Nothing that is flexible like this, which can also get banged around, will maintain pefect calibration.  Of coruse, one solution is to always add .3 degrees to the reading.  But I am not sure how it will fare at 45 degrees, 23 degrees, etc.