old man – Yhank you for the reply. In actuality what you are saying is not universal. Yes, what you are saying is the shittah of the Minchas Kohen (which was the prevalent minhag in Germany and Hungary), that all the calculations are calculated backwards from the actual tzeis hachochavim.
There were however shittos that used a fixed sunset and 72 minutes after sunset, regardless of the visual stars. This was the prevelant minhag in Galicia, and is followed today by Satmar, Belz, Klausenberg, and others. By the way, the simple reading of the Mechaber in Shulchan Aruch seems to follow this view.