Metamaterials involve small details that create challenges in their numerical analysis. As commonly practiced, homogenization of such complex structures may simplify and facilitate their numerical solutions. However, homogenization should be performed carefully to avoid excessive modeling errors, especially for finite structures. In this paper, we present accurate homogenization of three-dimensional metamaterials involving split-ring resonators (SRRs). Electromagnetic characteristics of finite SRR structures are found by rigorous optimization via genetic algorithms, while necessary numerical simulations are accurately performed by using the multilevel fast multipole algorithm. The results demonstrate the promising effectiveness of the approach for realistic metamaterial structures.