Rayleigh-Taylor instabilities are caused by features that affect shock velocity. These features can be statistically measured by radiography. We designed a precision radiography (PR) system that measures X-ray opacity variations in National Ignition Facility (NIF) ablator capsules to 10-4. Quantitative interpretation of the PR data is challenging and is the subject of this paper. The PR opacity power spectrum (PS) must be related to the NIF surface PS requirements (commonly known as the "NIF curves"). This relationship must be calculated for each specific shell. The compounding factors include X-ray spectra and spot size, detector resolution, shell diameter, coating thickness, dopant and impurity levels, and the coherency status of interface roughness between different layers. In this work, we developed a useful tool to quickly compute the NIF opacity curve (more precisely referred to as NIF "OD [optical depth] PS reference curve" in this paper) for any partially coated NIF shells or nonstandard developmental shells. This allows more rapid feedback on the quality of shells using only partially coated shells and enables benchmarking between the opacity (measured by a radiographic instrument) and surface roughness (measured by an atomic force microscope).