I would have to disagree here. Recommending that the squat be limited to 90 degrees due to your aforementioned criteria is putting a bandaid on the problem (ie. improper form, muscular imbalance, inflexibility, ego and overloading weight, etc.). Doing this will only further the underlying cause and allow for it to go unattended to. Without proper attention this has the potential to lead into to debilitating imbalances and injury. What should be done is the individual should seek proper hands on advice in order to deal with the problem instead of just bandaiding it. As a side note when improper form, imbalances, inflexibility, poor motor control, and ego, as limiting factors, come into play the squat is often affected the moment hip flexion is initiated.