I know it is not accurate, it’s just an unperfect example.
Let us go to a more extreme case.
If there is a farmer who owns 100% of the spaces and has 100 plots, the decentralization of the network is 0%.
If there is another farmer join and also has 100 plots, the decentralization of the network is 50%.
If there is a third farmer with 100 plots join, decentralization will become 66%.
There is a small guy with one plot joined, what is the decentralization for now? A quick calculation will be like 66.25%.
If there are 100 small guys with one plot joined in the network instead of one person. The decentralization will be approximate 77%. It is very close to another big guy with 100 plots in (75%).
In the other case. If there are 100 small guys in the network and there are no big guys exist. The decentralization of the network will be 99%.
That is how decentralization calculated. The number of individual farmers doesn’t matter. The number of farmer keys and pool keys doesn’t matter. The only thing matters are the number(size) of plots. The decentralization is not based on how many nodes there are in the network, but how the distribution of net space.
In order to attack the network, over 51% net space and the fastest timelord are necessary. HPool is a risk because they use third-party software to control all the spaces that join the pool. Official pool protocol is safe because the pool will not have access to anything that belongs to an individual farmer. So pools won’t able to use the space to get any advantage or attack the network.
Ideally, with official pool protocol. Even all the farmers join the same pool, it will not affect the decentralization of the network, because the individual farmer as a person has full control of their plot while the pool has 0 control over those plots.