Okay, so my initial impression from someone who knows nothing about this subject:
Archaeologists around the world agree with the year 586 BCE as the fall of Babylon based on archaeological and historical evidence, but the Watchtower Society made up the date 607 to coincide with their 1914 date that C.T. Russell predicted based on his measurements of the Great Pyramid in Egypt.
The 2520 number is based on the idea of "seven times" a day for a year (7 x 360 = 2520). The reason why we use the number 360 instead of 365 is because the Bible says 360 days are in a year (thus, 360 x 7 = 2520).
Now if we do the correct math using the starting point of 586 BCE and add 2520 years consisting of 360 days like the Bible uses, we arrive at the date 1908.
If we use the fictional date of 607 BCE, adding the same 2520 years consisting of 360 days each, we arrive at the date 1887.
Am I doing my math right?