You also have to figure in livestock. If all you have are a few chickens and some rabbits it's not too hard, but once you get into the bigger animals it's much harder to calculate. If you want to keep a steer on your place or a dairy cow, you are talking about quite a bit of pasture to support that animal especially if you want to grow your own hay and straw. One of the best ways to figure this out is to look at your neighbors. What do they do? Most of them probably buy hay and straw but you might find an old timer that is more self sufficient and will let you learn from him.