In previous work, we have assessed the effects of a wide range of systematic errors (Steiner et al. 2010, 2011) and found that only two are significant: the uncertainty in and the uncertainty in the spectrum of the Crab, which we use as our flux standard (Section 2). We take uncertainty in into account by repeating the analysis for and and then averaging the spin distributions, weighting them equally. We incorporate a 10% uncertainty in the absolute X-ray flux calibration (Toor & Seward 1974) by broadening our distribution in using a Gaussian kernel with 5% width. Finally, an additional 2% broadening is used to account for the small variation introduced by adopting a different choice of Comptonization model (Steiner et al. 2010).