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Far from the diffraction limit, and with negligible dispersion contribution, the source phase-space area scales as ${\epsilon }_x{\epsilon }_y$. At fixed photon flux, source-point optical functions and spectral conditions, brightness is inversely proportional to that product. The leading estimates therefore giveUsing the coupling-partition refinement in part b consistently gives about $275$ instead. Both estimates assume comparable current, photon flux and insertion devices. They would cease to apply once photon diffraction emittance or other source-size contributions become important; the question specifically excludes that regime.