I have found this article from the marvellous industry journal copyediting.com considering acceptable error rates for editors extremely useful for sharing with clients and setting expectations. Although editors/proofreaders do tend to be perfectionists, we are human, and it’s good for us and our clients to remember this.

If a piece is full of errors, even a 99% accurate editor will leave some errors behind.

Read Adrienne Montgomerie’s article, “Error Rates in Copyediting” here.