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Doesn't anyone RTFM? The first paragraph of the Battery Information section says to LEAVE IT PLUGGED IN. That's the owners manual's all caps, not mine.
"Remember, a connected Model S is a happy Model S."
Yup, 55% is best for the battery if the car will not be used for a few days or longer. I don't think you can set the Model S or X to charge to a point lower than 50% unless there's been a firmware update for that.