Everyone’s curious about this. The honest truth is that more art than science. Ideas come from everywhere, but ultimately the decision about what makes it into a cycle comes down to me (ceo), David (cto), and Ryan (strategy).
How we structure our work and teams at Basecamp
Jason Fried

Would seem to me that this is where leadership is defined. It’s about your gut and experience and is honed with time. I would submit that it is these decisions are what makes or breaks the process overall. That and the manner in which you stitch the cycles together.