Dougg -- and whose "years" are you using?

It was 538 BCE when King Cyrus (Ezra in 1:1-4 and Ezra 5:13-17) gave the decree to rebuild Jerusalem. Then in 517 BCE Darius gave another decree to rebuild the temple (recorded in Ezra 6:6-12).

This is a difference of 21 years.

You said you did your dating "back to" 416.5 years before year "0" BCE. Well, using your erroneous dating neither Darius OR Cyrus OR Artaxerxes' decree which was in in 445 BCE (Nehemiah 2:5-7) -- not 416 BCE.

So your dating is somewhere, somehow "messed up."

NONE of the decrees fit your dating schema.

Not sure how you did your math but I'd lay odds you aren't using Jewish dating to do so.

And everything that Sarah tells you, listen to her voice. Genesis, 21:12