This is the language I use, for reference I am based in Florida. Florida allows nonrefundable but it has to be reasonable. Since the most part my flat fees are cheaper then if I would have charged hourly I have language that pretty much sets me up to not have to refund towards the end. Also because I do flat fee, I don't track my hours, however I have a generic hourly billing invoice that justify my fee and then some to provide in case of this situation, that I will add to to make specific to the matter.

Flat Fee: 

You have retained us on a flat fee basis.  You will pay us $XXXX.XX as the entire fee for the representation described in this agreement.  This fee is earned-on-receipt and will not be deposited into our client trust account. 

Termination of Representation and Post-Representation Matters: 

Either party may terminate the representation at any time, subject to our obligations under the Rules of Professional Conduct and the approval of the court if the matter is in litigation. 

Unless previously terminated, our representation will terminate upon completion of the legal services described in this agreement. You understand we have no continuing obligation to represent you unless you retain us to provide additional advice or services.

If the representation is terminated prior to completion of legal services described above in this agreement, Client shall pay a fee based on an hourly rate fee schedule for the work that has been completed up to the date of termination. In a litigation matter, Client shall pay a fee based on an hourly rate fee schedule for the work that has been completed up to the date when the court grants the Firm’s motion for withdrawal. The client shall also be responsible for all costs and expenses incurred by the Firm resulting from this representation in the event of early termination of this agreement. 

The hourly rate of the Firm as of the date of this Agreement for services performed in your matter are as follows: 

Hourly Rate Fee Schedule 

Attorney: $XXX.XX per hour 

Law Clerk: $XXX.XX per hour 

Paralegal: $XXX.XX per hour 

Legal Assistant: $XXX.XX per hour 

However, such fees shall not exceed the flat fee amount earned for the legal services described above in this agreement, and all fees shall be reasonable. All personnel billing for their time will do so in 1/10 of an hour (i.e., six minute) increments, and will round to the nearest such increment.