It's worth bearing in mind that this debate over 606 v 587 is not new. In the October 1st Watchtower 1904 page 296 (RP 3436) Russell published a letter, which said (in part);
"Dear Sir, … You count the seventy years Babylonian captivity of the Jews as beginning with the overthrow of Zedekiah, Judah's last king, but I notice that "Bishop Usher's Chronology," … based on "Ptolemy's Canon," begins that seventy-year period nineteen years earlier … this in turn would make those times end nineteen years later than you have reckoned – in October, A. D. 1933, instead of October, 1914. "
Russell's reply was,
"We know of no reason for changing a figure: to do so would spoil the harmonies and parallels so conspicuous between the Jewish and Gospel ages. "
He then goes on to explain the damage it would do the their other years 1874, 1876 as well as 1914 and concludes with the statement,
"All this confusion would result from an abandonment of the Bible narrative in favor of Ptolemy's Canon"
So Russell did not chose 606 because of any empirical evidence, he chose it to support his pre-conceived ideas of parallel dispensations in his "Divine Plan of The Ages". And that's how it's been ever since.