QS is so black boxed, it is almost useless. That being said, take note that QS applies differently based on the level of competition. The more competitive the keyword, the more important QS becomes.

Best way to think of QS is how to do I make G the most money. If my bid is high but CTR is low, I won't. If CTR is high but bid is low I won't. You need to be somewhere in between these.