The 30% rule is a fallacy- the idea that there is a certain amount that can be changed to adequately transform a work is nice, but it ignores the neuances of what is really involved in a fair use analysis. Use of 100% of something may still be fair use. Use of 1% of something may not. It depends on the nature of the work, the actual portion utilized, the ability of the original copyright owner to exploit the work and create their own derivatives...it is unfortunately not a terribly usable standard for artists who want to make derivatives, since there is no truly objective standard.

Thanks for the link to the alternative view - I'm very much on the fence about appropriated art and my 10 years as an intellectual property lawyer don't help in that regard!