Intellectual Property Law
Intellectual Property Law Questions? Ask a Lawyer
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No, you don't and it's probably a better idea not to even mention it's an "adaptation" of a copyrighted work.
I don't think there is a "bright line" such as 10%, I believe that was the amount the court used at one time to decide if it was a "fair use" in certain circumstances.
Rewriting is not the same as "adapting". To be safe you should really take the original concept and then write a "new" work.