Relays are considered as a feasible way to increase coverage and capacity of cellular networks. However, optimization of relay-enhanced networks is still a matter of research. This paper proposes a simple geometrical approach to derive the optimal long-term relay downlink transmit power with the aim of maximizing the average spectral efficiency in a relay-based cellular network. In the authors' model, a User Equipment (UE) is served by the network node from which it receives the highest power. The spectral efficiency at the UE location is maximized when this serving node is also the one that provides the best Signal to Interference Ratio (SIR).