For New York, Rabbi Moshe Feinstein maintained that the time between שקיעה and צה"כ is 50 mins (corresponding to walking 4 mil), unlike in Europe where it is 72 mins (see here):
... the calculation of the 4 mil between sunset and tzeis. His essential thesis is based on personal empirical observation. Rav Moshe recalled that when he was in Europe, they waited 72 minutes in following the shita of the R”T. Rav Moshe then claims that when he observed the sky in New York, New Jersey, and pashtus, the Catskills, the night sky resembled the 72 minute European night sky after only 50 minutes. Rav Moshe then deduced that the 4 mil of the gm in pesachim are actually 50 minutes in duration.
I have a few questions on this:
1) how does Rav Moshe get to the 50 minutes between שקיעה and צה"כ, is it an average of all times across the year?
2) in reality, there will be occasions where stars come out early or later than 50 minutes. He obviously weighed up the problem of making a blanket 50 minute rule. What are the implications of this?
3) at what latitude can the fluctuation of שקיעה to צה"כ not be averaged in this way?
4) is this method of calculating used anywhere else in the world?