I ran across this video a while ago, and it's great for all levels of classical guitar. I just wanted you to have another perspective on positioning and seating for the classical guitar, and Will does a great job explaining it, though he does seem like he's making it up as he goes.
You'll want to skip ahead to 5:30 in the first video and stop the second one when he starts talking about things that are...not..positioning. However, it is a great video and well worth watching as much or as little as you like. I apologize for the quality; there are other clips of this video that are much better quality, but they don't include the positioning part.