What is an acceptable percentage?
The percentage is a measure of similarity not a measure of plagiarism, the important thing is that your submission represents your own work, and any references to other people's work have been properly cited in your bibliography. If a 40% paper has properly cited all their sources, it would not be considered plagiarism (although it may fail for other reasons, for example a lack of original thought). If a 10% paper had a section directly lifted from another source without citation, it would be considered plagiarism and could result in a fail. All academic work is expected to draw from other sources, so some level of similarity is expected. As long as all sources are acknowledged in the accepted way, it would not constitute plagiarism. The question to ask is not "what is an acceptable percentage?”. The real question to ask is "Can I honestly call this my own work?”.