Upper panels: estimated best-fit scaling coefficients for synchrotron (α) and dust (β), for several masks, whose sky fractions are displayed along the bottom horizontal axis (see text). Lower panel: the probability to exceed, . The red symbols identify the mask from which the final scalings are estimated, but note how the latter are roughly stable over the range of sky fractions. Choosing such a large “processing” mask ensures that the associated errors are conservative.