When is the exact time of Mi-shayakir?
The exact time of Mi-Sheyakir is difficult to ascertain, and there are various opinions in its regard. Practically, the widespread custom in Jerusalem/Eretz Yisrael is to consider Mi Sheyakir exactly 60 minutes before sunrise throughout all times of the year, although other customs do exist. The widespread custom in the Diaspora is to consider Mi Sheyakir either from when the sun is 12.9 Degrees from the horizon, or 11.5 degrees from the horizon. Practically, each person is to follow a calendar that contains the listing of his city, and follows the Halachic opinion that he is accustomed to follow. There is no known Chabad custom in this regard.
 See Piskeiy Teshuvos 18:5
 This is dependent on various factors such as that 1) It must be a clear day without clouds and without snow, and without a bright moon or other street lights. 2) It is to be measured in a flat plane or on a mountain and not in a valley. [See Ashel Avraham Butchach 58; Nimukei Orach Chaim 58; Toras Chaim Sofer 58:2; Igros Moshe 4:6] 3) We no longer have Techeiles today to compare the white to. [See Aruch Hashulchan 18:9; Birchas Habayis 37:37; Shulchan Shlomo 18:2]
 Some hold it is exactly 60 minutes before sunrise. [Kaf Hachaim 18:18 as Minhag Yerushalayim; Sefer Eretz Yisrael of Rav Tukichinsky that so is custom of Sephardim] Others hold it is 52 minutes before sunrise. [Nivreshes p. 43 as custom of Eretz Yisrael] Others say it is 57 minutes before sunrise. Others say it is 42 minutes before sunrise. [See Birur Halacha 18 and Beis Baruch on Chayeh Adam 11] Others say it is 35-40 minutes before sunrise. [Igros Moshe 4:6] Others are lenient to consider Mi Sheyakir from 6 minutes after Alos. [Peri Megadim; Rav Ovadia Yosef]
 Kaf Hachaim ibid
 See previous footnotes!
 12.9 degrees corresponds to the opinion of 60 minutes before sunrise in Jerusalem. 11.5 degrees corresponds to the opinion of 52 minutes from sunrise in Jerusalem. Some calendars measure from 11 degrees or 10.2 degrees from the horizon. [My Zmanim in opinion of Kovetz Otzros Yerushalayim]