Determine what you need to get the job done correctly, on time, and on budget. It is not a bad idea to build in a safety margin here. This is often referred to as "padding". For instance, one might multiply the estimated time to complete a task/project by say 20% to allow for additional time to deal with unexpected occurrences. The amount of padding usually depends on the certainty one has as to how effectively the resources can be used. One should keep padding to a minimum. Once the requirements have been set, it is important to verify that.
