Structured P2P overlays supporting standard database functionalities are a popular choice for building large-scale distributed data management systems. In such systems, estimating the number of answers for structured queries can help approximating query completeness, but is especially challenging. In this paper, we propose to use routing graphs in order to achieve this. We introduce the general approach and briefly discuss further aspects like overhead and guarantees. Categories and Subject Descriptors C.2.4 [Distributed Systems]: Distributed Databases; H.3.3 [Information Search and Retrieval]: Search process General Terms Algorithms, Management, Performance