The Nancy Grace Roman Space Telescope Coronagraph Instrument (CGI) is a high-contrast imager, polarimeter, and spectrometer that will enable the study of exoplanets and circumstellar disks at visible wavelengths (~550 - 850 nm) at contrasts 2 - 3 orders of magnitude better than can currently be achieved by ground-based direct imaging facilities. To capitalize on this sensitivity, precise flux calibration will be required. CGI, like other space-based missions, will use on-orbit flat fields to measure and correct for phenomena that impact the measured QE. However, CGI does not have internal lamp sources, and the potential hardware solution of adding a diffuser did not meet performance or cost requirements. Therefore we have developed a method to perform flat field calibrations using observations of extended planetary sources such as Uranus and Neptune, using a combination of rastering CGI’s Fast Steering Mirror (FSM), tiling the planet across the field of view (FOV), 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 (WFC3), in WFC3 filters approximate to CGI’s Band 1 and Band 4. The simulations are performed over the un-vignetted FOVs for CGI’s direct imaging (DI) and polarimetric modes. We model QE effects in 3 different spatial frequency regimes including 1) high spatial frequency detector pixel-to-pixel QE 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 CGI’s FOV. We show that this process can correct for these QE variations to the required precision of 0.6% per resolution element with >20% margin in both the imaging and polarimetric modes in Bands 1 and 4 using either Uranus or Neptune.