[manicennui]

At most companies the stakeholders barely know what they want. They provide a vague idea of what they want, and you give a wild estimate (referred to as a Silly Wild Ass Guess). Knowing all of the details of the problems that one is going to encounter in their old legacy system is basically impossible without spending a bunch of time up front, and you always discover more when you start writing code, so it is best to just start and iterate until you have enough of what the stakeholders want and you move on to the next disaster project.