Did they fix Momentum duration? Last I checked, Momentum's duration was acting
extremely oddly. Your equation isn't accounting for the fact that the total duration of the Demonic buff is reduced somewhat by your passive (not including Meta) haste level, since it's 5 + EBChannelTime, nor is it taking into account early clipping of EB. Blind Fury also makes it
substantially easier to fit in 2 Death Sweeps, as it actually gives ~1.5s bonus to the duration of Demonic currently. All of that is likely a bug, but even without the bug, your equation won't hold up except in the special case of having no haste from gear (which is irrelevant, since you're trying to calculate the amount of haste needed for a second Death Sweep, which probably will not be 0%)