[zozbot123]

> That is, we give you $15k per year, but start taxing you the very first dollar you earn rather than only taxing income about , say, $20k.

Yup; even in formal models of "utility-maximizing non-linear taxation", it's actually very common for optimal clawback rates to be fairly high for lowish incomes, because (1) this allows you to have a decent UBI for people who literally don't have any other income, while at the same time (2) any money you claw back early is money you won't have to pay out or claw back higher up in the income distribution, and this allows you to lower marginal tax rates a lot for the bulk of income earners, which is good for incentives - especially long-run incentives on skill acquisition and the like. Think of the break-even point where you're getting nothing on net and have to start paying into the system - you don't want that income level to be too high, or else the whole thing would become both unfair and infeasible to fund!

In the real world, UBI is mostly about slashing complex paperwork, and preventing marginal clawback rates as high as 100% or perhaps more(!). But a clawback rate of even 60% or perhaps a bit more, is in fact quite appropriate.