The reading of Jeremiah 29:10 as 'at Babylon' rather than 'for Babylon' makes the meaning much clearer so it simply refers to the place of occupation or exile which the Jews would remain in until the seventy years for their captivity would expire soon after the Fall of Babylon, their captor. It really is quite easy.
So you would agree that the Jews who were being told this would have been captive "at Babylon" 80 years rather than 70 (under WT chronology).