Thermodynamic methods to stabilize nanocrystalline alloys via grain boundary solute segregation have become increasingly common, due to their reliability and relatively simple design space. Recent progress in this area has revealed the need for spectral information which captures the full distribution of environments in the grain boundary network. However, the few proposed methods to extend existing spectral segregation models beyond the dilute limit are themselves non-spectral or are not atomistically informed. In this talk, we present a thermodynamic model that captures the spectral nature of both the segregation energies and the interaction energies. We then describe an atomistic, physically motivated method to measure the full spectrum of grain boundary solute interaction energies in a general polycrystal. Additionally, we present a case study on the results of this method for Al(Mg), and compare them to the results of hybrid Monte Carlo / Molecular Statics simulations. And lastly, we discuss the potential of this approach for the rapid measurement of solute interactions during non-dilute grain boundary segregation from available interatomic potentials.