Here is the clipped sigma 5 histogram. It does appear quite different.
Yes, but it doesn't demonstrate what I was talking about. The unclipped histogram shows populations for -1, 0, and 1 that are very close for the sigma=5.0, but are radically different for sigma=0.5. The 5.0 allows for simple interpolation; IOW, if the difference between P and Zc were 10,000, and the population of value 1 was 5000, you'd know that the blackpoint was about 1 ADU off. This is where my first, mistated criterion comes from, now that I think of it.