at this point, I'm more interested in "available" than use. My birds have 30 acres when they (routinely) ignore the elctric fence. 5 acres +/- when they don't. But when a hawk makes an attempted snatch and grab, all 70-80 of them congregate in an area of maybe 2,000 sq feet or less until they feel safe again (usually a day or two).
I expect some judgement calls in this, but whether my birds get credited 5a or 30a, its more than 1k sq ft per bird, so immaterial. If someone had a few thousand sq ft of a poultry netting run for a flock of 8, and one bird routinely "flew the coop", I would encourage them to select an answer based on the flock's *intended* management - secure, less than <1,000 sq ft ea, whatever time allowed.
Hope that helps!