A magnetic environment model of the moment distribution in ferromagnetic alloys is applied to the Ni-Mn alloy system. The variation of spontaneous moment with composition is fitted and, from the parameters obtained, neutron diffuse scattering cross sections calculated with the further assumption of dominant first-neighbour interactions. These compare well with previous neutron measurements and much better than calculations based on any of the simple Mn spin-reversal models. The internal ratios between the three first-neighbour interactions obtained from the model also compare well with two experimental determinations from the spin-wave stiffness constant.