I found this link in your link...
http://www.backpackinglight.com/cgi-bin/backpackinglight/xdpy/forum_thread/9378/index.html

Very nice chart on clo vs temperature and activity. It's based on the skin temperature of 85F. I see where you might have come up with 0.8" of clothing loft for light activity at 40F. That's a bit sketchy because it includes the surface effect, which sort of comes with the shell layer but can only be counted once and is reduced by wind chill effects. But it is a very practical chart in a way as it provides some extra allowance for the wetter conditions you often get from 20F to 40F.