enemy rank probablities???

Has anyone ever figured out within the games’ programming how the ranks of enemies are spawned within campaign mode? It’s understood that the higher the difficulty, the more likely enemies are to spawn with higher ranks, but has anybody ever figured out what the ratios are? I know people have figured out the recharge times for elite shields based on rank, for example.

I’m sure the ratios are out there somewhere. With that being said, what the ratio is when the skull is enable that gives everyone a promotion so everything is walking around towards the top of the rank hierarchy