We present a robust methodology to improve the accuracy of relative calibration in bi-directional reflectance distribution function (BRDF) measurements using 99\% nominal reflectance Spectralon (P99) standards. Conventional single-angle calibration approaches can introduce significant uncertainties because Spectralon exhibits non-Lambertian scattering behavior. In this work, the BRDF of P99 was measured over a comprehensive range of incident angles covering the upper hemisphere and subsequently parameterized using Zernike polynomial expansions. By integrating these measurements to derive the directional-hemispherical reflectance (DHR) and scaling the results to NIST-traceable DHR values, a calibration scale factor independent of measurement geometry is obtained. When applied to measurements of particulate samples, this approach substantially reduces systematic errors and yields accurate and consistent absolute reflectance values.