Look at the stickies in the wheel and tire section. They get into technical descriptions about what each number means and how it relates to the whole. It's very useful, even just as background knowledge.
Here's what your wheels will look like. Yours would just be 15mm further out. He went with 2004 STi wheels because they're 5x100.
http://www.ft86club.com/forums/showthread.php?t=34789