For several decades, proxies for solar activity (such as the f10.7 and Mg II indices), rather than direct EUV measurements, have been used as inputs for ionosphere/thermosphere models. For that reason, it is important to quantify the capacity of each proxy to reproduce a given band of the EUV spectrum. Now that the EGS spectrograph onboard TIMED has been continuously operating for more than 6 years, it becomes possible to carry out a detailed comparison between solar proxies and various spectral lines, both on short and on long time-scales. Here, we use information theoretic criteria to quantify the amount of information that is shared between 9 proxies and a selection of important spectral lines. This analysis reveals in a synthetic way which proxies are most appropriate for reproducing specific spectral bands for modelling purposes, and how this can be interpreted in terms of physical processes on the Sun.