Q. If one holds that Shabbos is out 72 minutes after shkia, may he wash for shalosh seudos until 40 or so minutes after shkia?

A. Mishna Berura (299: 1) rules that you can wash for shalosh seudos until a half an hour before nightfall. Shaarei Tzion (ibid. 2) explains that this is due that the mitzva overrides the prohibition of eating after the shekia.
Horav Shlomo Miller’s Shlit’a opinion is that there are a number of variables that one must consider to obtain a correct answer, such as location and time of the year. In this location during Tamuz, it should be closer to thirty minutes after the shekiah.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a