Quantitative interpretation of X-ray reflectivity measurements from liquid surfaces requires methodical accounting of the effects of diffuse scattering from thermal capillary roughness. In this paper we discuss how this requires careful attention to the shape of the experimental resolution. These considerations, which are essential for measurement of the intrinsic structure factor of a liquid surface, require knowledge of the liquid surface tension. The paper closes with a brief comment on the most sensitive method for extraction of the surface tension from measurement of the off-specular diffuse scattering.