Good agents ask for 15 percent, no reading fee. Good agent should know what kind of material is in demand. Learn which magazines have the best editors and offer the best contracts. Paul Aiken, as a writer, fired his agent and took over his own contract negotiation. Now offers seminars in contract negotiation.
Publisher actually has a range of contracts depending on the relative innocence of the author.
Your job is to return the document with a list of all desired changes, your wish list. Publisher accepts some, says others can't be done. You say, let's negotiate. Publisher says, can't. Listen to the rationale, don't debate the issues because the publisher is much better prepared than you are likely to be.