Quote - Edit: That 10% myth is actually true, just not for 3D modeling. It works for recipes, and some other products you find in stores. I know of a company that orders a certain popular brand of picante sauce, they add a few of their ingredients, put it in their jars with their labels, and sell it commercially. So yes, the 10% rule does apply somewhere, just not in the 3D world.

It's not true in any aspect of copyright law.  Food recipes and formulae are generally considered trade secrets and have little to do with how copyrights are handled.  Aside from that, simply mixing food products together and reselling them doesn't actually have anything to do with either, it has to do with whether it's legally okay to resell that food item.  Likely that example of yours involves some form of permit from a government agency (USDA I suppose) and permission from the original manufacturer.

In copyright context, any derivative work requires permission, there is no percentage rule.  The only exception is "fair use", and resale or free redistribution definitely does not qualify as "fair use".  Read:
http://www.copyright.gov/help/faq/faq-fairuse.html#howmuch

http://www.copyright.gov/fls/fl102.html (fair use in general)

My Freebies