It is an interesting question. I am not sure if there such thing as 100% decentralization, but there should be some threshold to be at least decentralized enough to further facilitate the progress.
I will have to think and research more on this. But to give a quick answer, to derive some metrics I would consider how difficult it is for the system or network to fall under control of one entity or a group of entities. Then I would also consider distribution of influence. Combining these two I think we can come up with a meaningful method of measuring levels of decentralization.
I would categorize Hive as at least decentralized enough to keep the network secure. But in comparison to other networks, Hive would be much more decentralized.