Rabbi Reisman had a shiur regarding this a few years back. IIRQ he said there are basically two shitos, Shitas Hageonim and Rabbeinu Tam. Both Shittas hold that the time is divided into three parts, vadai yom, safek yom safek lialah, vadai lialah (tzeis).
Rabbi Reisman then went on to discuss the timing in America according to Reb Moshe. He said that Reb Moshe held tzeis for shitas hageonim is 35 minutes after shkiah and 50 minutes after for Rabeinu Tam. According to this, vadai yom is until approximately 8 minutes before tzeis therefore mincha would be until 27 and 42 minutes after shkiah respetively, then it would be bdi’eved until shkiah. If one were to not hold y Reb Moshe’s calculation and use the standard 42 and 72 minutes then IIRQ the vadai yom is until 18 minutes before tzeis according to Rabeinu Tam (I don’t remeber the timing for shitas Hageonim).