Sun elevation for ISS visibility

[Cristiano]

I use several web sites to search the visible passes of the ISS over a given location.
Then I calculate the Sun elevation as seen from the ISS and I get surprisingly low elevations; for example, I get ISS magnitude= -3.8 and Sun elevation at the ISS= -19.9 deg.
I suppose that the atmospheric refraction is very high, but what's the lowest Sun elevation to consider the ISS outside the Earth's shadow?

Thank you

 

[mfb]

You can look up the ISS height and the radius of Earth and calculate it yourself.

-19.9 is just at the edge of visibility.

 

[Cristiano]

Please, would you post the formulas?