im new so this wont get much attention but i have been gathering data and have worked out roughly what you need to finally get that rank you always wanted SHOTGUNS!!!! 
  
K/D is a big factor but they head of zombie development tweeted out that its not revives or  head shots its your highest  round and how many downs you got in that games so if you get to round 40 with one down and do that over and over your are going to rank up really quick unlike someone who goes to round 20 with 15 downs  
  
BONE: 1-10 K/D with 1k Kills 
2BONES 10-20 K/D with 2-4k kills  
SKULL 20-30 K/D with 4-7k kills  
SKULL AND KNIFE 70-200 K/D with 7-15k kills  
SHOTGUNS!!!!!!! 200+ K/D with 15k+ kills  
  
everybody says they tally mark has something to do with it it doesn't that is simply how often you play zombies. you can rank up with 1-4 ticks blue or no blue eyes dosen't matter. 
  
please help people put and place you kills, downs, K/D and rank so people can aim towards an average to level up 
  
ive got 
  
KILLS: 6092 
DOWNS: 53 
K/D: 121 
HIGHEST ROUND: 71 (BURIED)  
RANK: SKULL 
  
ive just started a new account to measure how many and what i need to rank up as my main account i meesed around on and have a K/D lower than 80 
  
add me on xbox: EricGherkin and i will help you work out your K/D and kills you specificity need to rank up