The approach of taking a blown out frame and a completely dark frame is valid for calculating DR, but there are inherent assumptions in that calculation (equivalent response between the frames, etc.).
Shouldn't the the inherent assumption of dark frame and white frame calculations be that everything between those two extremes can be recorded by the sensor at one time?
Correct me if I'm wrong again (probably am), it's "black" when all pixels record 0 and white when all pixels record 255, in other words there are a sum total of (2^8) brightness values per pixel that can be recorded.
But we aren't concerned with that, we're concerned with how dark it has to be to black out pixels and how bright it has to be to white out pixels... in one scene.
I can go stand my wife up to the wall next to a window on a bright day. If I expose for her, the window will be overexposed white. If I expose for the outside, she'll be underexposed black. If I expose for the median, there will be a little of each (histogram shaped like a U, I guess), and detailed brightness range between those extremes is the total DR my camera can record. Is that what you mean by usable DR?
Put another way, do these methods suggest anything about where highlight and shadow clipping begins in a high DR scene exposed for the middle tones?