Need help understanding lens shift-

[Dieselboy]

I'm trying to design (currently only in my head) the avaliable spots to postion a projector in my room and I'm a little confused as to what the 'parameters' are for where the projector can be postioned. How do I determine the maximum amount of 'off center' a projector can be mounted? Specifically I'm concered with how close I can mount it to the ceiling-

I'm going to mount it on a shelf built in the wall and I can go down a max of about 7inches below the ceiling. I know that LCD type projectors offer more mounting options than DLP, but I like the price of the HD20 Optoma. So when I go to projectorcentral.com and use their calculator for this projector it shows that at my throw distance of 13ft. there's a number of 8.6inches between the projector and the top of the screen- is this the minimum or maximum distance the projector should be mounted? Or does this number represent something different entirely?