I just recently dealt with this issue last week on my 46G. Are you setting up on carpet or hardwood/linoleum? If you're setting up on carpet, remember that the carpet will "give" a little and therefore the shim thickness should be judged accordingly. I have a stand thats curved in front so I used a 3/8" piece of Luan plywood that I scroll sawed to match the contour of the stand for its full length. it only needed to be an inch deep to do the trick.. and it worked beautifully....tank is level!
good thing for you is that it is tilted back and not forward... you want to make it level so the water looks level, but if it has to lean one way, back is the safest. I sympathize with your situation though... all i wanted to do was setup my tank when i realized that the floor wasn't level....just another frustrating hurdle to overcome, but it's a pretty quick fix once you get the height of the shimm sorted out.
Ryan