How are zombies rankings calculated in Call of Duty: Black Ops 2?

Jimmy Zielinski (former Treyarch Director BO1 & BO2 Zombies) made the following tweet back in 17 Oct 2013:

"The Ra k is ive", it ris and lls bas on ho ll y pl erall. I is ba on w ma dow ver y fin nd. Low nd dow cou aga y mo th dow in lat nds..."

This slightly hidden message actually means:

The Rank is alive, it rises and falls based on how well you play overall. It is based on how many downs versus your final round. Low round downs counts against you more than downs in later rounds.

Further reading:

Ranking System Confirmed!

Jimmy Z Explains Rank