It's probably a good thing to have the chicken math "attack" before the birds arrive so you'll have a place to put them all once they're out of the brooder. By building something so big, there is more of a risk of "getting it wrong" in some way but your design looked really solid, and so did your grasp of the "whys" that drive the design. As long as you plan for the worst reasonable predators and the flightiest birds you foresee wanting to have, you should do well. Your whole property will probably look much nicer if you build one large structure than if you were to build a small structure here and there. The only thing, is if you wanted to have an area for quarantine, you might still want to have a smaller, separate area on the opposite side of the property, downwind from your established, larger pen.