I think Leftback's numbers err on the pessimistic.
I agree with Josh - we'd probably be looking at around 1,500 if we were doing well, and if we were challenging for the title we'd be looking at 2,000.
In the Conference I think you'd be looking at the same for a mid-table season. Title-challenging up there, we'd be looking at 2000+ most games, which is similar to what I'd expect us to get for most FL games if we ever returned. Remember, for the really important games we can pull in about 3,000, even at this level.
It probably seems a bit sad all this, but the obsession with crowd sizes is understandable. When you're not doing well on the pitch it's the thing you can cling to so you can reassure yourself things are better than it seems. So on the one hand it's nice to know we can get those crowds, but on the other it makes it all the more frustrating when we don't.
It'll be interesting to see what bearing the new ground will have on crowds.