“As far as your assertion of the dates coming from Torah and neviim-I think that -if you count the years of “malchus jehuda’ until the churban bayis rishon- you will find that it does not reach till 420 either.”
See this link that does the math and biblical citations:
http://www.dafyomi.co.il/kerisus/jewishkings.pdf
Mods, please let this through. You can see that it’s from a Torah teaching web site.