Here is a post that has help me align my XL bands and it works:
Crisis averted!!! I called this morning and spoke to Dewayne at BGE HQ Customer Service. He was extremely helpful and guided me through the steps to level out the top. It was extremely simple to do, even for someone who didn't understand the way these things are put together.
So basically here's what was the problem and what I did. The first problem was that the lower band was not wrapped around evenly - it was lower in the rear than in the front. He explained to me that the lower band should come right up to the edge of the lower gasket all the way around the lower half. So I loosened the carriage bolt on the lower rear band and GENTLY tapped along the band using a block of wood and a rubber mallet until the band was placed evenly around the whole bottom. Then I tightened the band. From that point, the underbite had improved significantly, but there was still a bit of unevenness along the planes where the gaskets met and it still did not pass the dollar bill test. In fact, the gap on the rear side between the gaskets was slightly worse, but I wasn't worried because I knew there were a couple more steps to
Next step was to adjust the front to back placement to fine tune the sitting of the top to the bottom. There are two slotted acorn nuts on the top part of the hinge (above the spring) on both sides of the top that needed to be loosened so that I can 'wiggle' the top into a centered position. So I loosened all 4 nuts, wiggled it into position, then tightened. Now the top was centered properly (no under or overbite), but there was still unevenness on the plane of contact - in other words, it still failed the dollar bill test.
The last step was to somewhat repeat the first step (loosening the carriage bolt) except for the upper band. Wherever there was the largest gap between the two gaskets, I GENTLY tapped the band upward which essentially pushed the lid downward by it's own weight. Once I saw that the upper band was positioned in such a way that the entire top part of the egg sat nicely and evenly on the bottom, I tightened the carriage bolt of the top band and vuala! It was fixed. Dollar bill test passed all the way around the egg. No more underbite (or overbite).
I'm happy as a clam!
Good Luck,
Brian