I think Charlotte's rules are mostly designed to make her happy. She knew exactly what she was getting into before she married Mr Collins, looked at it from all angles, and decided on a coping strategy. She has almost decided to be happy no matter what. Unlike Mrs Bennet, who doesn't have enough sense to realise that her life could have been different, she has taken on the role of Mr Collins' wife in full knowledge that other marriages are different. But she has taken on the role willingly and clear-sightedly, and is making the best of it.