I think I've tracked it down, Quendi, thanks to your clues. The "Standard Inscription of Nebuchadnezzar" which talks about all his extensive building work. But there is this little section:
"Four years (?), the seat of my kingdom in the city ..., which ... did not rejoice (my) heart. In all my dominions I did not build a high-place of power; the precious treasures of my kingdom I did not lay up. In Babylon, buildings for myself and the honor of my kingdom I did not lay out. In the worship of Merodach my lord, the joy of my heart (?), in Babylon, the city of his sovereignty and the seat of my empire, I did not sing his praises (?), and I did not furnish his altars (i.e. with victims), nor did I clear out the canals." - George Rawlinson, The Seven Great Monarchies of the Ancient World, Vol. 4, 2007 BiblioBazaar paperback edition, p. 171.
I've not seen that one before!
The statement in the BTGHF book - dug out my old 1997 WT-CDROM and got it!
*** bf 372-3 18 Getting Out of the Midst of Babylon ***
Jehovah God is thus proved to be an accurate Timekeeper. If we follow his system of counting time, according to his written Word, we shall make no mistakes in our calculations. We cannot therefore go along with the chronologers of Christendom who date Jerusalem's destruction as occurring in 587 B.C. and who thereby limit the desolation of the land of Judah without man or domestic animal to merely fifty years. Almighty God decreed that the land had to lie unworked, uninhabited for seventy years in order to enjoy a relatively perfect number of sabbaths, that is to say, ten times seven sabbaths. Had the land enjoyed less than this perfect number of seventy years, it would not have enjoyed its full number of sabbaths. God's decree could not be broken or set aside, and, true to his decree, the land of Judah did rest uninhabited seventy years, from 607 to 537 B.C. In his own Word Almighty God, the perfect Time Measurer and Counter, says so.-2 Chronicles 36:19-23.