Yeah Jack, those two rules-of-thumb are in close agreement on your 56/720 example, and also with the S&V graph, which shows about 11'. On your first point, note that it's the "real" resolution of the program material, which isn't increased by upscaling, which limits the observable resolution. For example, the S&V article states in discussing an example(beneath the first graph): "...so a high-def program on a 720p HDTV-or a 720p program viewed on a 1080i or 1080p HDTV...", indicating that the 720 program material is the controlling factor with regard to resolution on the 1080 set.
