For helping with judging NLC candidates (are they NLC or not) I have a set of formulas to calculate the minimum altitude (in km) of the candidate given an observed altitude (in degrees) of the candidate (using known stars, like Auriga, as reference points) and the date/time (giving the Sun's altitude).
A typical observed altitude is 10 degrees, largely unaffected by refraction and a typical result is 70 km (ruling out any other type of cloud). The formulas are limited to only work in the horizontal direction (azimuth) of the Sun (that is under the horizon), but this is not a problem in practice.
Reasonable assumptions are used, except one: no atmospheric effects like refraction and scattering are accounted for.
Are NLCs only visible to the naked eye if directly exposed to sunlight (the Sun directly visible by the NLC)? Or is scattered light enough to make them visible?
What about refraction? Light coming to the NLC goes through near vacuum when it is close to the NLC so the refraction must be less than for an observer on the ground, but what is the exact figure (when the Sun is at the horizon as seen by the NLC)?