Surely it depends on the extent to which the time the work will take will change depending on external factors. A fixed fee for a first draft of terms and conditions is very different to a fixed fee for litigating a dispute to trial. I obviously use an exaggerated example to illustrate here but it is fair to say that a fixed fee can simply be a solid estimate of the time a matter will take multiplied by hourly rate (at least that's how we arrived at fixed fee estimates at my old firm).