"He who marches out of step hears another drum." ~ Ken Kesey
The rule is 30% but that really means nothing. I have seen photos that I would say were almost not recognizable of the original. They lost in court. Just tell the guy for the low price of a graphic designer you can have original art for him and he will never have to worry about going to court over it. If he still acts up. Tell him to move on.
10%, 20%, 30%, 50% I dont think it really matters..... If they can prove that someone took their exact design and knocked it off then that is grounds for a lawsuit. I have actually been involved in this 2 different times in the last 18 months and really my advise is to try and not copy anything at all..... If someone wants to recreate something then we talk them into starting from scratch. For this particular situation I would make your client sign some paperwork releasing you from the design, that your contracted to only handle the print side of it and all legal consequences will fall on them. But if he is talking lawyer this, lawyer that to me that is usually a sign to run as fast as you can.
Our artwork approval form has the following clause, which must be confirmed. It may or may not be watertight, but its a helluva start, I think.-Upon requesting any and all third party images (company or corporate logos, trademarks, or any other copyrighted images) to be reproduced on your goods, you verify and acknowledge that you have been granted permission to use these images by the copyright holders of the images. Mr. Tees is not responsible for unauthorized reproduction, as you are wholly responsible for the necessary permissions of use. ***We use this mostly in situations where we are printing a sponsor logo for an event tee, or what-have-you. We still use judgement when things smell fishy, and could be a potential problem....like the kid wanting me to print Nike logos last week.....
30% smaller text, of course.