What are the curve radii for the curved slope pieces? Does anyone have a table with the curve radii for the curved slope pieces?  

If not, does anyone have a good idea for measuring a third point on the curve?  

Since I know the front and the back "coordinates" of the curve piece, I need a third point to get the radius. I was thinking about making some sort of jig, but I don't have any good ideas.
These curves have been approximated by the contributors to the LDraw project. You can download free software to view and use the part files, from which you can approximate other points on the curve. For example, here is an LDraw part file shown in "wire-frame" mode:



LDraw parts are drawn in increments of the "LDraw Unit" (LDU) in which a standard 1x1 brick is 20 LDU wide by 24 LDU high (not including the top stud). By examining the part in an LDraw viewer, I was able to establish the coordinates of some of the points on the curve: