ItcheSrulik – The common practice to daven Mincha after Shkiah and maariv before 72 minutes, is done for the most part completely Lehalachah.
It is done based on the Minchas Kohen’s shittah in Rabbeinu Tam, which was followed in Hungary (not in Galicia), that did not hold of 72 minutes. Rather Tzeis is based on 3 small stars, and the bein hashmoshos beginning 14 minutes before that. Therefore if you assume that Tzeis in NY is about 50 minuts after Shkiah, you can daven Mincha 30 minutes after Shkiah and maariv afterwards 50 minutes after Shkiah. In addition, the Maharam Shick holds that you can daven Minchah during Bein hashmoshos, and this psak was widely accepted in Hungary.
For those who hold with a strict Fixed 72 minutes (Satmar, Klausenberg, etc.) daven Mincha at approximately 45 minutes after Shkiah with Maariv following at 72.