Dunlop CG4/5 alignment tool

[Laurenzo]

Chaps I wonder if you could help me out here, I have just laid my hands on a Dunlop tool that measures camber and castor etc.
 
Now that is all very well and good, but what do I need to do to simply measure my camber front and back. The tool looks very simple, and I guess that you place the level across the rim top and bottom and adjust the scale to get the reading?
 
However in my case the tyre walls hold off the level, so the question is should there be a bolt on accessory to allow the tool to touch top and bottom of the rim?
 
A picture is worth a thousand words!:Hammer