Can Any body help?

I know it sounds daft. But after using an 8mm Sigma fisheye for a while, I'm trying a Sigma 10-20.I use a Canon 450D and I want to shoot at 20mm. The reccommed settings are 55 lower rail and 100 upper. However, I would have to turn the camera plate around to be able to get to this setting? Furthermore, how true are the suggested settings, using my 8mm I found some of the settings out on the bottom rail after using the "Grid" as parallax experiments.

Any help appreciated!