Wherever the border line is of the land in that area the Russians bought from the Turks should be where the end of this stupid war is.
Bought? Not much buying went on, unless in the context of peace treaties after substantial bloodshed.
There were at least a dozen wars fought over this region since, say, 1600 (and I am probably underestimating this). Some of this was more or less claimed by the Turks, if only through alliances with the Tatars or others. Some was conquered by natives, Ukrainians, even if in service to the Tsars. And any such historical line, even if it can be determined, would be arbitrary and irrelevant to the current residents.
There is no simple historical line of any sort. The clearest and most legitimate (according to the political mores of the last couple of centuries) is the also-arbitrary line that held at the time that Ukraine voted for independence. That is the line that the Ukrainian nation (which exists, now more than ever) claims as its right.