Our rule is a bit non standard. With no use case there is no place to write a functional requirement. If it ain't written, it won't get built, tested or anything else. You'll see some books and experts say "logon is not a use case" maybe because it doesn't accomplish a business purpose. But for us it has to be a use case or nobody will ever write it.
I always recommend Cockburn's "Writing Effective Use Cases". If you're new to using them - and sounds like you are - he can give you a lot of comfort and show you a wide range of choices. Other authors can give you a lot of anxiety - do it like this or your project will fail! and get you hung up on formalizing the format and enforcing the rules and reviewing for the right table of contents structure. Yuck!
Know your audience, write clearly just for them. Relax, have fun, come back here for more comments.
A good question is never answered. It is not a bolt to be tightened into place but a seed to be planted and to bear more seed toward the hope of greening the landscape of the idea. John Ciardi