A shallow-layer model for granular flows down inclines is completed with a closure relation for the macroscopic bed friction obtained from micro-scale, discrete particle simulations of steady flows over geometrically rough bases with contact friction. Microscopic friction can be different between bulk particles and with particles at the base, where the latter is systematically varied. When extending the known friction closure relation to be a function of both bulk flow and bed properties, surprisingly, we find that the macroscopic bed friction is only weakly dependent on the contact friction of the bed particles and is predominantly determined by the properties of the flowing particles. Implications for constitutive modelling and possible experiments to better understand the bulk rheology are discussed.