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The Nancy Grace Roman Space Telescope Coronagraph Instrument is a high-contrast imager, polarimeter, and spectrometer that will enable the study of exoplanets and circumstellar disks at visible wavelengths ($\sim$550--850~nm) at contrasts 2--3 orders of magnitude better than can currently be achieved by ground or space-based direct imaging facilities. To capitalize on this sensitivity, precise flux calibration will be required. The Roman Coronagraph, like other space-based missions, will use on-orbit flatfields to measure and correct for phenomena that impact the measured total effective throughput. However, the Coronagraph does not have internal lamp sources, therefore we have developed a method to perform flatfield calibrations using observations of extended sources, such as Uranus and Neptune, using a combination of rastering the Coronagraph's Fast Steering Mirror, tiling the planet across the field of view, and matched-filter image processing. Here we outline the process and present the results of simulations using images of Uranus and Neptune from the Hubble Space Telescopes Wide Field Camera 3, in filters approximate to the Coronagraph's Band 1 and Band 4. The simulations are performed over the Coronagraph's direct imaging and polarimetric modes. We model throughput effects in 3 different spatial frequency regimes including 1) high spatial frequency detector pixel-to-pixel quantum efficiency variations, 2) medium spatial frequency "measles" caused by particle deposition on the detector or other focal-plane optics post-launch, and 3) low spatial frequency detector fringing caused by self-interference due to internal reflections in the detector substrate as well as low spatial frequency vignetting at the edges of the Coronagraph's field of view. We show that Uranus and Neptune can be used as astrophysical flat sources with high precision ($\sim$0.5% relative error)