I don't know about the Dalton line, but there was a halfway decent piece that got posted on /r/nfl last week that said after 32 starts you can put all QBs into four categories based on averaging their AY/A (average yards per attempt) and how it compares to league average. With a couple minor exceptions its groupings held pretty true. League average AY/A is 7. Andy Dalton's career AY/A is...7.

So if your average AY/A after 32 starts was something like 20% or 15% above league average (8.4 AY/A 8.05) or then you'd fall into the "superstar" or "star" category, respectively. If you were hovering around league average or a little bit under then that's all you were, "average". And if after 32 starts your average AY/A was more than -5% league average you could safely be considered a "bust". If you do the math on Tannehill I think he's around -6% for his career, which would qualify him as a bust. The article was about Dak and Wentz so I'm not sure if I'm remembering everything correctly, but I think that's more or less how it went.