Hi Ray,
Nice graphs, well done. One thing not quite captured here, and I don't know how one would do it in a completely analytical way, is the fact that when we dither our images the FPN (I'll just call it that for now) is smeared when the stacking actually occurs. Does it make it worse or better, not sure, haven't thought that far. But I do know that it would be better if wasn't there to start with!
Point two: Many years ago I was shooting the breeze with Paul Boltwood and Doug George at Doug's place. Paul had just recently won the deep sky competition set out by Sky and Tel mag (amateurs imaging the Hubble deep field to see how deep they could go), and was quite surprised that he had beat others, since he imaged from an obs attached to his house, in suburbia (Kanata) of a ~750K population center (Ottawa). He had only a 16" newtonian reflector and a camera he had made himself (this guy was very bright and handy, I can't imaging designing and building a custom CCD holder myself!). One thing he said was: going deep without really really really good flats, fogget about it...flats ultimately are THE limiting factor in how deep you can go.
Best,
EB
PS. I just found a link to short bio on him:
http://astro-canada.ca/_en/a2223.php