Back flat in the drops is on the extreme end of the spectrum. It also depends on anatomy: short-torsoed riders have more difficulty getting into that position (less availability to flex in the back, as opposed to just the hips).
Andy Pruitt recommends 30 degrees for this angle:
I stretch every day, attend yoga classes, practice on my own, and do a twice a week set of crunches and other resistance training. I can get my palms on the floor with my legs almost straight. Yet I can't hold a flat back position.
From a side-angle photo of myself on the trailer, in the drops, my upper back is 5 deg, my lower back is 23 deg, and the net angle from my hip to my shoulder is 21 deg.
But if you look at the photo of the High Road rider (I think this photo is going to set the record for most-reposts, so I won't reproduce it here), he's getting a lot of bend in his back: his lower back isn't especially flat. I just can't do that and still breathe.
But a lot of pros look like that. From Steve Owens on cyclingnews
A misconception that I want to point out is going to really bug a lot of people because they're going to feel at a total loss on how to find the most aero position. Each person's aerodynamics are different. What works for one person, might not work for another. A flat back isn't always best. In fact, we're finding that a rounded back is good for a lot of people, and it opens up the hip angle - typically giving you the ability to produce more power. That's one thing I'd caution people: be careful of how you close the hip angle.