In order to grind or file a semi-precise radius (between day 3 and 8 mm in radius) from square stock, I use a method of preparing three facets on each corner that remove the bulk of the material, and then I find it is relatively easy to just freehand the rest to create a reasonably nice, smooth radius.
This calculator helps you figure out the distances/borders of these facets. I generally just scribe the 45 degree facet lines on both sides of the stock, and then one line (the more distant one of course, since the other one would disappear after filing the 45 degree facet) for the shallower angle. That creates a good guidance for the first 45 degree facet, and then the other two facets are done by means of the calibrated eyeball doing its thing. Hope this helps you as it does me.
English