Epilespaul,
I have the same projector. What you are experiencing is the offset of the projector. The unit has an offset of 20% which means the image will be 20% of your vertical screen distance above the center of the lens. This is why tilting the back of the unit up brings the image down to where it needs to be. I have my unit ceiling mounted and have to point the front of the projector up towards the ceiling (opposite setup from yours). In your scenario, you will have to raise the back of the unit like your doing to account for the offset. At the distance your projector sits you should be able to have an image up to about 102.5". However, this is with the unit zooming the image. You will get a better picture if you can move the projector back and not zoom the image.
Doug