Apologies for my lack of precision. The "70 Years" I am referring to is the period related to the Watchtower's "2,520 Years". In retrospect, I should have referenced the latter.
Magum,
I wish that that Watchtower reference was more clearly expressed. We can see they are referencing the departure of Jews for Egypt. From the structure of the grammar, does the period commence when the people started to set out for Egypt or when they crossed the border, thus making at that point the "land where no one dwelt?".
I have located only a few relevant statements in the Society's publications to the start of the "2,520/70 Years", with the most relevant in the 1963 "Babylon the Great Has Fallen!", but for some reason it has gone the way that "forbidden books" disappear.
Ironically, the Society's illustrations of their "2,520 years" show the removal of Zedekiah from the throne as the starting point. But they have problems with that since Jerusalem was destroyed and Zedekiah removed in the 5th month (July/August).