Goodell DeVries Leech & Dann LLP - Jim Astrachan, Lexology; Copyright Fair Use: How Much Copying is Too Much Copying?
"...no plagiarist can excuse the wrong by showing how much of his work he did not pirate.” These words were written by Judge Learned Hand in 1936. His point was that a taking of someone else’s expression will not be excused merely because it is insubstantial in quantity when held up for comparison to the infringing work.
Years back a copyright defendant client related copyright lore as a defense to his actions. He swore up and down that copying was permissible as long as not more than 10 percent of the source work was taken. Many times that belief has been mistakenly repeated. Many of the older, bedrock, principles of copyright practice are worth repeating. Perhaps this repetition comes from being the teacher that I suspect is part of my DNA.
The “ancient” case of Harper & Row Publishers, Inc. v. Nation Enterprises, 471 U.S. 539 (1985) should absolutely disabuse anyone of this silly notion."