I sort of tried that -- only the correct formula in Euclidean geometry I use is Sqrt((xb-xc)^2+(yb-yc)^2+(zb-zc)^2). But no matter how many times I did it, I ended up with an answer that didn't work! I ended up finding a value by trial and error and by setting the F stop to 1.4 was able to clearly see the focal sphere of the camera, but the numbers didn't seem to make sense..... I'll mess with it some more...