There is no such 30% rule. Tests for copyright infringement are much more complex. And even if there were, a copyrighted work changed by a certain amount, so that it can by copyrighted itself, will still be a derivative work, subject to the original copyright.

Weird Al can do what he does, because it's satire, and there are exceptions for that as fair use and freedom of speech in copyright law. Try changing a Beatles song by 30%, in a non-satirical way, and see how far you get with publishing it...