All three depend upon various opinions and interpretations of the Rishonim and later Poskim.
The Gemara declares that there is a period of time named "Bein HaShmashos" which is doubtful whether it is still day or already night. Sometime during this time period is the transition between them, but is impossible to determine precisely. Therefore, this entire time span is considered questionable, and one must be Machmir to consider it day as well as Machmir to suspect it is night. This time period is described as lasting the amount of time it takes to walk ¾ of a Mil, between 13.5 and 18 minutes.
There are two primary opinions when Bein HaShmashos occurs, the Gaonim place it immediately following observed sunset while Rabbeinu Tam describes it a s immediately preceding the appearance of 3 medium stars 72 minutes after sunset.
According to the Gaonim, this span is relatively simple to pinpoint, from astronomical sunset until 13.5-18 minutes later is questionably day/night, and one cannot do Melacha then neither Friday evening nor Saturday. According to this view, 18 minutes following sunset on Motzei Shabbos one could consider it certainly night with all that entails, the only remaining issue is determining the precise time of Halachic sunset, which is especially complicated when hill block out the sun before it sinks below the horizon. However, this is generally a matter of a few minutes, and 25 minutes after astronomical sunset would be safe to do Havdala etc.
However, Rabbeinu Tam's opinion is much more complicated, as it has no clear starting point. Furthermore, the two reference points given of 72 minutes and 3 medium stars to not necessarily coincide, complicating matters further. Additionally, some Poskim calculate the 72 minutes typically while others utilize Zmanios, meaning 1.2 times 1/12 of the time between sunrise and sunset. Others calculate the sun's relative position below the horizon (and there too, some use 12° others 16° and yet others 20°). According to the most extreme calculation of this opinion, nightfall in the summer could be close to 3 hours following sunset.
Despite its complexities, this opinion cannot easily be dismissed. Many, if not an absolute majority of early Poskim support Rabbeinu Tam's position as the accepted one. This also fluctuates significantly between various locales; the custom in Hungary was to follow Rabbeinu Tam completely, while in Lithuania many accepted the Vilna Gaon's endorsement of the Gaonim's view. While we certainly would not suggest relying on Rabbeinu Tam's ruling to be lenient Friday night, many Poskim including the Mishna Berura exhort people to be Machmir for it, at least when the issue is a potential Issur d'Oraisa such as Melacha on Motzei Shabbos.
While it is difficult to give a conclusive ruling on an issue that has so many prominent Poskim endorsing such a wide variety of positions, I would suggest that while generally it is ideal to wait at least 72 minutes before doing Melacha on Motzei Shabbos or any other potential Biblical transgression, as this is the opinion of the both the Mechaber and Rema in the Shulchan Aruch, in case of need one could rely on the following moderate consensus position.
Both HaRav Moshe Feinstein zatzal and the Chazon Ish zatzal understood the primary factor according to Rabbeinu Tam as being the appearance of 3 medium stars. Consequently, Reb Moshe calculated the time of Havdala in New York as being 50 minutes following sunset, and the Chazon Ish reportedly generally observed the required stars approximately 40 minutes after sunset in Bnei Brak. He also proposed that if one can see 10 stars of any size, we can assume that at least 3 of them are medium.
While I am not familiar with the speed of dusk in St. Petersburg, this is something you should be able to easily observe yourself. Once three medium stars are visible, I believe you could safely rely on the combination of the Gaonim who consider Shabbos long passed, together with the Gedolei HaPoskim who opine that Rabbeinu Tam would already agree.