The answer is easy and implements common logic.  If a potential client is willing to sign an engagement letter that sets forth a maximum number of hours that SHOULD resolve their case, you can use an open-ended agreement with a defacto fixed fee.  Assuming the client is reasonable and responsive in a timely fashion, both parties will be satisfied and job will most likely be completed for the estimated price.  As is often the case, a client will be unresponsive or slow in providing information which adds more work time and increases fees.  It is for this reason that I prefer not to work on a fixed fee basis.  If you choose to work for a fixed fee, be careful to ensure that your engagement letter is very specific about what you are and are not including and have caveats to address client’s that are difficult to work with.