That's a nice concise little guide that should help people. I've been doing it since before I even had an arc but for some reason whenever I try to teach the math to someone I turn it into a book trying to explain something that is so simple after a few goes at it.
While I like your guide this ^ is just completely incorrect as
@Titris Thrawns pointed out. While it's one thing for a guide to have a debate after it or certain things be a matter of opinion, this however is an incorrect absolute fact of the game mechanics. In order for this to work as a guide it needs to be accurate or else it would only lead to confusing new players further. Please edit.
In case you weren't aware yourself though any GB level you donated to you can still donate to until the GB levels. This is only not possible if: A) The person deletes their account/city B) The person deletes the GB C) The person disconnects the GB from the town hall.