Mrs. Congeniality gave us this: Easy Math for 586/587, which is a stripped down version of AlleyMom's KISS method.
The upshot of this reality is that for 607 BC to be correct, a king or kings must have reigned for 20 collective years BEFORE Jehoiachin's exile (a.k.a. Jeconiah) in 597 BC and be missing completely from the record.
The reason for choosing this date as pivotal is two-fold, (1) the number of years between the exile of Jehoiachin and the destruction of Jerusalem is known and was entirely covered by Nebuchadnezzar's rule, and (2) the time that passed from Jehoiachin's release until the restoration is known.
Why could not Jehoiachin have been exiled in 617 BC as the WTS asserts? Because, if that were the case, Babylon was destroyed in 559 BC instead of 539 BC. 539 BC is a derived date, and the same pieces of evidence that fix that year also fix 597 BC as the year for Jehoiachin's exile. Therefore the missing 20 years had to come before Jehoiachin's exile, if there ever was 20 missing years of Babylonian rulership.