thanks M&T - Stevec35 recently posted a really nice image.
fwiw, the maths is in this spreadsheet:
http://www.iceinspace.com.au/forum/s...d.php?t=151000
my f4 system is within 5% of fully sky limited at 60 seconds since the read noise of the camera is below 2e. I have been using 2 minute subs for convenience, but lose a bit of dynamic range in doing so.