The concept probably won't have any suitable metrics to be used as a gauge so I don't think there is any absolute answer to this.
Oh, it is easy:
Let
n be the cardinality of the smallest set of actors that are potentially able to collide for committing a successful adversarial behavior against any subset of users by somehow violating their rights for joining/continuing/exiting the network deliberately and safely. We can trivially define a decentralization index as:
DI = 1-n-1