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Marcel,

When I did this recently, I just made up my own rules based on my own experiences spotting stars in twilight. The people who work on the open-source "Stellarium" project have clearly implemented some rules which may be founded on something more solid, so you could probably contact them --or just get a copy of the source code and find the rules they used. I don't imagine there are any reliable "scientific" studies of naked eye visibility of stars in twilight since weather conditions can change everything.

For atmospheric extinction near the horizon, I recommend this site:
http://www.icq.eps.harvard.edu/ICQExtinct.html.
See especially the tables at the bottom of the page.

Speaking of weather and stars, in the past few days I've been amazed by the blackness of the sky around the nearly Full Moon here in Chicago even when there have been broken clouds at low altitudes. Normally there's a a gradual transition from medium to dark blue and then black extending five to ten degrees around the Moon. That blackness right up to one degree from the Moon is usually a sure sign of very dry air (also dust-free) at higher altitudes. I was able to see third magnitude stars just a few degrees from the Moon a few days ago, even with the light pollution of Chicago. That's really unusual, especially in August, when the skies are usually murky with haze.

-FER

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