Yeah, I think you're a bit confused there geetee1972.
You set your chain length up as per the bottom of vinnyeh's first post and all Shimano documentation – largest sprocket and largest chain ring plus 2 links. This means you won't snap your rear mech off when running biggest front and back whatever length of cage you have.
The non-issue comes when running small sprockets and chain ring at the same time. The mech has to take up the links that are not now running over teeth. But as experience has shown, there isn't a problem with a medium cage despite what the teeth ratings would suggest.
I think it's safe to conclude that SRAM and (even more so) Shimano are conservative with the teeth ratings of their rear mechs. This is probably to allow for people who set up their chain length with more than the specified 2 extra links (either because they don't know any better or so thay can remove a link if the chain snaps and the pin is lost?).