Understanding the composition of the paints used on works of art is crucial in attributing them to the artist or workshop that created them, as well as to devising strategies for their preservation and restoration. Much current research is focussed on the development of non-invasive spectroscopic methods, enabling in situ paint analysis without the need for sampling. However, whilst a lot of work has been carried out on looking at the composition of artists' materials qualitatively, little has been performed to analyse them quantitatively. In this work, we investigate the use of Fourier transform Raman spectroscopy, in isolation and in conjunction with complementary fibre optic reflectance spectroscopy, to look at the spectral changes that occur with varying the paint/binder ratio in model historic lead-based paints. When used in conjunction with multivariate analysis, we demonstrate that it can be possible to obtain a semi-quantitative estimation of the composition of a paint film from its Fourier transform Raman spectrum, given a set of suitable reference spectra recorded from paints produced within the workable mixture range. This represents a tentative first step towards a semi-automated and quantitative method for analysing cultural heritage objects, which may help boost our understanding of their history, and enable conservators to make more informed decisions so as to ensure their preservation.