The 1200 mm image:

The result here turned out better that I had expected. Again, a nice clean in-focus line running along the x-axis. You can also see how the smaller checkerboard has more squares that are going out of focus. By rendering larger images, you should be able to put a reasonable estimate on the actual DOF over a range of focal lengths.

I did find it a bit odd that FireFly rendered the DOF along the entire length of the x-axis. In a real photograph, the DOF should look circular with items near the center in-focus and items out on the edges not so. Guess that's a limitation of FireFly. I also found it interesting that the DOF is apparently the same distance in front and in back of the focal point (in this case the origin of the plane). Usually the DOF is greater behind the focal point. It may be interesting to try a short animation using frame by frame distance values by this method. If I work one up I'll post it.