Runs that are 10 square feet per chicken are the standard here for outdoor space. Those runs are bare of vegetation. Chicken yards with 100 square per bird feet usually only have limited amounts of green on them, especially in bad growing conditions. Depending on the climate 1000 square feet per bird may be needed to provide a large part of their needs. Certainly the space your planning on, is a place to start at. Watch and think on expanding their range if necessary. Plan on feeding them if the conditions, say winter, isn't providing enough.