There has been a lot of discussion about this accessory recently. That being said, and as I have recently lent mine out and had to give a tutorial.lets review. First, regarding accuracy; the tool I received was spot on, coupling this protractor with a rail will not diminish its accuracy. Second, this application is not to "hold" a rail at an angle, but to "set" the angle in relation to the edge the protractor is set to. This does not mean that this operation is meant for or with clamps. As a rule regarding this unit, if you would use a clamp for the operation not using the angle unit then use a clamp with it. Finally for me, I do not understand why someone would alter this adjustable unit to a fixed 90 degree. It seems to me that there are much cheaper squares available. Hope that helps, Chris