In the end, this is the magic of compounding interest. If we have an existing database installed, it can tell us not only how much space it currently consumes, but also how quickly it's currently growing. If not, we can start with a medium size and substitute a growth assumption that will cause the cumulative total to double in size every year. Remember, we begin by working with worst-case scenarios, and modify the numbers afterwards.
What if we don't need compounding interest because our expected growth is linear? It's always easier to start with too much space than to add more later. If you know your table count will rarely change, users will not increase in number, or data streams are relatively consistent, feel free to ...
