Singer’s proposal in that article isn’t _quite_ that, though it may be that he just didn’t think it through carefully enough (or deliberately simplified in an article intended for general consumption). He proposes that the fraction you give of your _total_ income should be, if you’re in the top [10%, 1%, 0.1%, 0.01%], [10%, 15%, 25%, 33%], producing discontinuities at the boundaries of those groups. I suspect that if pressed on that point he’d be happy to go with something smoother.
Singer’s proposal in that article isn’t _quite_ that, though it may be that he just didn’t think it through carefully enough (or deliberately simplified in an article intended for general consumption). He proposes that the fraction you give of your _total_ income should be, if you’re in the top [10%, 1%, 0.1%, 0.01%], [10%, 15%, 25%, 33%], producing discontinuities at the boundaries of those groups. I suspect that if pressed on that point he’d be happy to go with something smoother.