Would it be easier if vaults came in fixed sizes? It would be trivial to measure the free space ratio.
As my (not fully confident) understanding goes, chunks are assigned to sections by their XOR distance from the section ID, which assures that chunks are uniformly distributed across the sections. If the number of vaults within a section falls within a narrow range, then it would make sense to make each vault responsible for roughly the same number of chunks (well, a given number of chunks; vaults can still come in different sizes, just not arbitrary sizes).
When a new vault is allowed to join, it would be assigned the smallest acceptable size the section allows at the moment (e.g. 32768 chunks) and then it would be given chunks until it filled up to the same level as the other vaults in the section. At that point, it could request its size to be raised (e.g. to 65536 chunks), or it could stay as it is.
If vaults are assigned a known number of chunks, each section would know how full the network really is just by looking at its own vaults. It would be easy to decide when to accept a new vault to join (e.g. we want 80% full, 20% for cache.) It wouldn’t be a big mistake if a new node is accepted by “accident” because it could still be denied a size upgrade at a later point.
Sorry if I’m talking rubbish. I tried to find relevant information but couldn’t figure out the exact details about some things. For example, is the Safecoin economy still relevant for the free space calculation? I think I read something about that, but I’m not sure.