I agree wholeheartedly with Gregg recommending against the use of any electronic keystone adjustment. It is guaranteed to destroy your image quality, even if you can't see it. A far better way to deal with this is to insure that your screen is perpendicular with the projected beam. Use the offset optical control first after making sure the projector is level in both planes. If this is not enough, then angling the screen can be used. This usually requires angling the bottom of the screen away from the wall slightly until the geometry is correct.