I would offer some clarification, at least based on my experience with Carl Strong, and anecdotal reports of other cyclist's experiences with quite a few other builders:

"The key is picking really good builders" ...period. If they're good, you don't need to "know just what you want" in any particularly elaborate, technical way; you don't need to know anything about size or geometry or Lateral Stiffness/Vertical Compliance/yadda-yadda-yadda... All you need to know and to convey to the builder is what kind of riding you like to do (fast? slow? long? short? smooth asphalt? dirt path? etc) and what if anything you would change about your current bike. In the interview/dialog with the builder, they'll ask the questions they need to ask to figure out what "you want".