Slope, s, that maximises the luminosity for given total mass and surface X_H for Z = 0.008. Left: Slope, s, as a function of surface X_H for total masses ranging from 1 M_⊙ to 18 M_⊙. Right: s as a function of total mass for X_H values ranging from 0.01 to 0.7. The plus signs indicate estimates from our structure models, while the dashed black lines represent our best-fit formula from Eq. D.1