How should I store my rMBP for a month?
You are overthinking this as there is nothing special about the lithium battery that requires special storage considerations.  Just store it like you normally would; I put mine on a bookshelf to hide it from the inevitable human curiosity of the dog walker/plant waterer when I go on extended trips.

Your lithium ion battery is going to lose capacity whether you use it or not because the biggest factor in the longevity of a battery is age not how much it's charged.  Now, the amount that it loses over the month is going to be negligible; you won't even notice.

I posted an answer that is relevant to this question:  MATH FINDS TOP CULPRIT AS LITHIUM-ION BATTERIES AGE

Just leave it plugged in over the month and you will be fine.