The "fair use" defense to copyright infringement is actually pretty subjective and definitely not a bright-line test. What does it mean to be a fair use? No one can tell you exactly, at least not in the abstract. It all depends on the particular facts. What I can tell you, is there is no "10% rule."
The fair use defense is part of the copyright laws-Section 107 of the copyright laws to be exact. In short, whether a copy is a fair use basically turns on four things:
- Is the copy being sold?
- How "artistic" is the work?
- How much did you copy?
- Did you impact the original author's income?
So really, fair use is very fact specific. But if you just think about it in terms of those 4 questions, you can probably figure it out for yourself.