Line-strength H_βO versus [MgFe]’ index-index diagram for Milky Way globular clusters. Filled symbols correspond to the globular cluster sample of Usher et al. (2017), color coded by their [Fe/H] metallicity. The FASTAR model grid prediction is shown in the background in grey. Labels indicate ages and metallicities across the grid. The equivalent width for data and models is measured at the FASTAR 2.51 Å spectral resolution. FASTAR model predictions reproduce the observed data without systematic offsets, suggesting that the well-known zero-point problem of evolutionary stellar population models can be at least minimized with state-of-the art isochrones.