There are lots of details, but I would vote that it should be fairly simple, and connected to few facts beyond your IRS income. There should be no incentive to

There are lots of details, but I would vote that it should be fairly simple, and connected to few facts beyond your IRS income. There should be no incentive to rent a bigger house, or move to an expensive city, beyond the usual salary/rent tradeoffs. (And no cliff-edges, so perhaps it's max 15% of the income above twice the poverty line?)