Accurate normalisation of high energy x-ray scattering data may be accomplished when the composition of the sample is known and the experimental corrections are either negligible or have been rigorously applied. However, for samples with unknown compositions, a priori normalisation is impossible. For binary rare earth phosphate glasses we demonstrate an iterative method for determining both the appropriate normalisation as well as determining the ratio of rare earth to glass former. The isotropic scattering is compared for several estimated compositions and fitted to the measured data over different Q-ranges. For high energy x-ray data (E=115keV) where the attenuation and multiple scattering corrections are very small the sample composition is insensitive to the Q-range used. We demonstrate this method for a rare earth phosphate glass sample and compare the results with established methods of atomic composition determination. The accuracy and applicability of the method is briefly discussed.