why do you need to figure out what that number is?

Just use the storage as intended - for your daily needs.

Daily needs do not include a content mirror for pirate bay, or archiving every twitch stream in existence.

Use it in good faith, and if people think you're using too much they will send in a salesman, not the enforcing agents.

If I can really trust their statistics, it's fine. But without knowing the number that's a nagging worry at the back of my mind. Because if we look at the new numbers they think are reasonable, 1TB or 5TB per user, that's not enough to hold normal backups of my data.

Sending in a saleman instead of an enforcing agent isn't much better if the salesman wants me to pay $100 a month.

You used the example of 100TB in another comment. I think 100TB would be a reasonable standin for unlimited when it comes to a single user. But would it be so hard to just say that somewhere? Maybe throttle uploads after that point? Instead you have one guy using 20TB per account and one guy using 2PB per account and they both get screwed by the change.