I live in Serbia, we have something similar that the government gives us to calculate the tax from additional income. https://frilenseri.purs.gov.rs/kalkulator-poreza.html

I would image that other governments have the same thing. Or at least easy to find policies.

Would be awesome to see a centralized base of all the governmental tax policies with calculations. Good for nomads especially. You could then extract all sort of data from this. Especially if people start using it.