Reply To: Rabbeinu Tam's Later Shkia and Shabbos (and Mincha)

i recall as a youth davening at a large chassidic Bais Medrash in BP erev Shavuos and they started mincha when I could no longer discern the maple tree leaves outside from the darkness. As an adult I became hardcore about actual shekia, and also learned from Rav Premock that RT tzeis is variable based on time of year and lattitude, and that fixed 72 is a widely held error.