It’d have to be 1:1 usable to be fair, so if you share 400 GB, you’d get a right to PUT 100 GB.
As I mentioned in other topics, we have to remember that each PUT-ed file is copied fourfold so if you upload 100 GB you’ll take up 400 GB from the network.
There are other important details which were mentioned elsewhere (I think also in this one). For example, if you delete 50 GB of your content but the network in v1.0 can’t delete, you’d still have to share the peak amount of space (400 GB) even though you now you’re needing just 200 GB of physical space (50 GB * 4 copies)…