Thanks, but as mentioned I tried scaling up the amount of light from the whiteboard (either via size of plane, or incandescense/ambient intensity), but to get a decent reach, the image gets washed out.The other problem with scaling it much bigger than the object is that it starts to illuminate more areas that should be in shadow. The picture shows the plane just big enough to get a visible shadow, and both prolems can be seen.

For anyone else learning FG like me, in the end, I faked what I wanted by adding a very low intensity spotlight. (Turning on 'Secondary Diffuse Bounces' also helps, but doubled render times.)