I'm not sure what you're getting at with 89 vs 90%.
Oh, I was saying that someone just under the poverty line (90%) gets a full prebate on their "flat-distributed" % tax. Someone just over it (89%) get's absolutely no benefit. This is my argument for graduated taxes.
In most EU countries, one does not pay taxes at the first XX euros his/hers yearly income, than he pays some low tax at the next YY EUR, and than higher percentage for next ZZ euros he earned (if he earned that much) and so on until some large percentage that is usually set for all income over 100K EUR for example.
So it is gradual for everyone. And if, say, the no tax limit is set at 15K EUR, there is not much difference for guy earning 16K euros and 15K euros per year. The first one would pay maybe 10% of one thousand euros while the second won't pay any taxes.