> That's an option, and I'd be interested in how the math works

The math works because the derivative of a constant is zero, it's that simple.

> That said I don't think not decreasing benefits is an important requirement. But net income (benefits + earned income - taxes) should be strictly increasing with earned income, and probably at a rate of at least 50% but ideally higher than that up to some reasonable definition of middle class income.

Exactly, it doesn't need to be fixed, we could also have decreasing benefits, simply with a much lower rate of decrease (ideally the rate of decrease should be the marginal tax rate for this income). But in practice such a system would be much more complex than a flat benefit system for little gain (the more limited the decrease rate is, the closer to a flat rate your public spending would be).