I honestly wouldn't even know how to measure this -- and really, isn't the deep ball more about timing, placement and touch than arm strength?
I'll say this, though. I watched Ryan Tannehill rifle some passes into tight windows and I definitely think the guy has a live arm. How does it compare to Joe Flacco? I have no frigging clue, but he can definitely put enough mustard on the ball to get it where it needs to be.