With the rapid proliferation of drones, there have been numbers of opportunities and challenges in drone application research area. One of the highlighted topics about the drones is constructing an ad-hoc network of drones and utilizing each as an access point (AP) to build an instant network infrastructure. However, there has been no research about drones optimal position improving network service quality of end users. We analyze the drone-based network infrastructure in terms of the network capacity allocation, and propose a drone formation algorithm that determines the 3D geographic location of each drone. Since each drone is hovering in the air while acting as an AP, its altitude must be also considered for the efficiency and performance of wireless communication. We assume the signal propagation model of the drone to be the shape of a sphere and define the relationship between the altitude and the coverage area of the drone. With this property, our algorithm finds the appropriate coordinates of drones one by one. Through the mathematical analysis, we show that the proposed algorithm can make drone-based network infrastructure provide reliable and user-satisfying network service.