Meditating on a software development task and picking a random number is a guess. There are other methods that provide equally effective methods of planning that do not include guesses.

I developed the Gnome Narrative method over time as a way to both limit the amount of time I spent defining and illustrating projects and as a way to communicate logical designs to non-technical decision makers. Gnome Narratives have slightly more details than a napkin drawing but not much more. Often the diagram can stand on it's own without much description.