Not to confuse the issue, but although that's what the literature says, I think 90 birds in a 30x30 run sounds like a nightmare. I wouldn't go so far as to call it inhumane, but it's going to be darn crowded. You'll need to be prepared to put in a lot of work keeping it clean, have a very efficient deep litter system, or enduring a god-awful stench and possibly frequent bouts of disease--not to mention stress and the possibility of cannibalism and all the other issues related to overcrowding. I think a third the number of birds (or less, of course) would be much more workable. With only ten or less, you might actually be able to keep a nice grassy pasture going in that run, for instance! The thing is, the more you try to intensify the whole thing, the more stress on the animals, the more work it requires, and the more problems you have to deal with. Stuffing that many animals into a tiny a space is, well, frankly, just not natural--by which I mean that nature can't cope with maintaining it, therefor the more you have to step in and interfere to do so.
I have a space that I'm guessing is about 1.5 times the size of yours, maybe a little bigger, for about 25 to 30 birds. Including a covered run and two small "pastures" (mostly dirt) that I normally try to rotate. They use to be confined to just the run, but then I made the two pastures, because I felt it was too crowded, and more space was healthier.
Generally, the more space you provide for the number of birds, the cleaner and easier to manage the whole thing becomes. So if the question is how much space for a chicken (or for that matter, a duck) my general answer is going to be "as much as possible, because the more the better."
Common sense is more useful here than numbers IMO.
That's my 2 cents worth. Best of luck!