I'd like a variant on this: X% of income *above the poverty line*, no time limit, disappears upon death. Administered through the IRS, it's a line on your tax

I'd like a variant on this: X% of income *above the poverty line*, no time limit, disappears upon death. Administered through the IRS, it's a line on your tax return and thus automatically adjusted for income. (And I'd do the same thing with child support--x% up to $y.)