100% minus fixed height - overflow

I am having problems with my "100% minus fixed length" site. I know I'm not the first and I certainly won't be the last. I have read some articles and looked through a few posts on this forum but i cannot seem to grasp the concept.The page can be found at http://www.lms-rpg.com/releases/v2-1/index/modern/login_success.html the problem is that content of the page goes past the 100% which i thought i had implemented correctly causing a scrollbar to appear on the window. i can set the body Overflow: hidden but this just cuts off the part that was causing the scroll bar. let me know what you think. All comments are welcome! thanks