A clear description of the local atomic structure as a function of the composition is essential to guide discovery and development of new metallic glasses. The efficient cluster packing model suggests that solute atoms avoid nearest neighbor contact as long as possible, and this is a key feature of the metallic glass structure and stability, while the random packing model does not have such restriction. We propose to critically test these models by carrying out a systematic survey of the atomic structure of the binary Ca-Cu glass system, from 15 to 60 at.% Cu, by integrating combined neutron and X-ray diffraction data with ab-initio molecular dynamics and Reverse Monte Carlo. The number and type of atoms in the first coordination shell of both atomic species as a function of composition and the highest solute content where solute-solute contacts are avoided will be determined.