Golly, was I dumb or what: the 490 is psychological, not mathematical. That is to say, man goes through, on a macro level, a psychological adjustment phase. It starts out about 50 or 70 years prior to the 490's ending, as an exhaustion: man has tried all his ideas with whatever is then his concept of 'civilization', and they all fail. War ends the phase. Then a new one begins, as some other group of people or nation rises to fill the inevitable vacuum, and it lasts the remaining 70 or 50 years (approx. 120-160 total, 4 generations).
I can trace this pattern from Adam forward, though the first ending period is somewhat sketchy: I need to do a lot of homework on it.
Post-Cross, you can easily see the phases, since we have more data to work with. For example, the exhaustion of 630AD (which was 490 years after Aelia Capitolina, which came on a timeline exactly matching that of the 1st Temple destruction to the rebuilding of Jerusalem in Daniel 9). Then, Byzantium and Parthia exhausted each other, creating a vacuum which the Arabs then filled 8 years later (obeying Muhammed's 632AD (last) speech in Arabic being, 'go up to Jerusalem and kill all the Jews' -- you can't find this in published versions of his speech, but only in the madrassas, apparently). Sir John Glubb in his book, A Short History of the Arab Peoples made a point of this Arab invasion timing due to exhaustion, though of course he didn't say anything about the 490s or the Bible.
Each subsequent period has the same kind of exhaustion 'play', though with different actors. The next vacuum was filled with the Crusades. After that, the Reformation. And now we are in a new vacuum, so of course the Arabs arise again, and on time, just as they did to usher in the Crusades and the Reformation. That means we should be at war again, by about 2020, with the Arabs as the catalyst, much like the assassination of Archduke Ferdinand triggered WWI (due to the underlying patchwork of conflicting alliances, here the patchwork being alliances with and against, the Arab nations). China may come out the winner, this time.
I'll have to write a webpage on this, and make videos. I don't know if I can do it this year.