I really like this approach. The only problem I see with this is that the measure of center you’re using for the calculation would have to be chosen well. Maybe a weighted average towards lower incomes or using a median, but properly choosing the measure of center would be really important along with verifying that a linear increase or decrease in percentage is also the best solution.

I think figuring out the rate at which taxation should increase relative to ones income should be the first step, then mapping that onto a set of equations that could calculate taxes would be the best approach.

If you want some help with installing, I can try to help.

Why are you uncomfortable buying a used phone? I’ve only ever bought used phones.