Video-on-demand (VoD) service providers are intensely interested in transport, storage, streaming and caching in content delivery networks. Today?s 5,000-hour library may grow toward the 750,000-hour "Long Tai" movie and TV-series catalog. We propose a method to calculate how much of a library should be cached. Much previous work focused on theoretical caching concepts, or the dynamics of cache filling and reclamation. Our method explicitly considers the impact of the available video server equipment; we present a VoD design tool comprising a novel cost function, hit ratio estimation and heuristic.