The DILAGON: a DIY laser goniometer

[Cyrano]

... The knife edge is to be positioned at the diameter of the semicircle, and since we use the semicircle the scale itself does not require calibration as in case of a straight scale ...

A straight scale would not require calibration in the sense of using some physical object as a standard. Straight or circular, as long as the knife edge is positioned at a known and fixed point, a scale can be created. A straight scale would need to be sized appropriately for the intended distance to the knife edge -- but that is true for a circular scale, too. 

This diagram represents a fold-up template similar to the DILAGON.pdf file posted earlier in this thread -- but instead of a semicircular scale, this version uses a straight scale. To make this template into a goniometer, one would simply fold the diagram along the central horizontal line: