For athletes who are less proficient at the Snatch (i.e. non- Olympic weightlifters) 66-70% sounds about right. For people who are technically proficient, the snatch should be over 80% of the Clean & Jerk max. As for a ratio to back squat max I believe somewhere in the neighborhood of 50-55% sounds about right for someone who is skilled at the lift. Clean & Jerk however would be a much better predictor.