A couple of references to Adams zone system.
http://en.wikipedia.org/wiki/Zone_system#Digital
http://www.luminous-landscape.com/tu...e_system.shtml
I am still puzzled on how this relates to what you are doing.
Adams was working in monochrome. His system was to ensure correct exposure and development of film and print to match the tonal range of the scene to the dynamic range of the media.
In your case how do you know that part of the scene relates to grey level 3 if you have no reference to work from.
Using a device such as the Lastolite XpoBlance to get a reference would be more useful.