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It was a handwavey rule of estimation for dedup, handwavey because dedup scales on number of records, which is going to vary wildly by recordsize.

Additionally unless it's changed in the last six years, you should pretend ZFS dedupe doesn't exist.

Not in a stable release yet, but check out https://github.com/openzfs/zfs/discussions/15896 if you have a need for that.