For Fairytales, Fables, Folklore, Legends, and Myths-
As these stories are based on the above, which are public domain, what is needed is that it's fiction, and what myth, legend, etc it's based on.
For example-
This is a work of fiction, and based on The Judgement of Paris.
What you're doing there, is stating clearly it's fiction, and what myth inspired you. We don't need profit for it, as it's based on a myth.
For Titles in the Public Domain-
This is for stories based on book titles which are in the public domain. Categories where we have verified that there is no copyright of any kind, anywhere, and that it is indeed in the public domain, will be here.
Example disclaimer -
This is a work of fiction, based on Shakespeare's Twelfth Night.