For Earnest and others who may be interested, here is a link to a page that discusses how CTR came up with 606 as the date for Jerusalem's fall to Babylon. I think the link is written by poster AlanF. (Correct me if I am wrong.) And here is a Google listing of several other possible links to this topic.
The WT's 606/7 dating for the fall of Jerusalem to Babylon in Nebuchadnezzar's 18th year appears to be nothing more than taking 536/7 BC and subtracting 70 years, with the assumption that Jerusalem had to have fallen at that time. Thereafter, it represents a decision to hold to that conclusion in the face of history / archaeology, and the possibility that one has misunderstood the scriptures that were used to make the initial calculation.