MDG- thanks for the link. By the way- there is a discrepancy in those tables if you count the years of the individual kings- you get to 383-if you extrapolate the dates -it is 374- Maybe you can make some sense in this.
If you add the 40 years of shlomo’s reign and the 480 years before that- it would make either 903 or 894 years from jetzias mitzrayim to the churban bais rishon. The regular count we have is from seder olam rabbah (2nd century) and it would place jetzyias mitzrayim at about 1300 BCE. (2448 mybrias haolam). An earlier poster said that 586 came from josephus- this may be because josephus puts jetzyias mitzraim at a much earlier date (appx 1500 BCE),.Hence, it is possible that 586 did find its first proponent in josephus. well, now we can all go back and do soem more research.