While lanthanum hexaboride is an excellent instrumental broadening standard in most cases, the lowest angle diffraction peak corresponds with a d-spacing of ~4.158Å whereas many samples will exhibit peaks at lower angles. Instrumental broadening functions obtained from LaB₆ can therefore result in poor fits to diffraction data at low angles. In this work, it is shown that by using a suitable large d-spacing standard to constrain the instrumental broadening function during profile refinement, fit quality can be substantially improved for large unit cell materials.