I want to buy 20 Old English Silver Duckwings or Silver Leghorns. How much space will I need in the brooder until it is OK to cut-em-loose outside to forage? Is there a rule-of-thumb "sq.-feet-per-chick" number? It is obvious to me that I can section-off the brooder in the beginning because the chicks will be small. And, move the divider gradually until they have the entire brooder to run around in. For the two breeds I mentioned, at what age is it usually OK to set them free outside to forage on their own?