Well, it's not required, but highly encouraged for Jews to give 10% of your income to charity (not necessarily to the poor, but to any charity), since it is a Jewish custom that started with our forefather Abraham who gave 10% of his income to Malcheitzedek, who was a righteous gentile priest of G-d and all religious Jews follow this custom and give 10% of their income to charity. If you are poor yourself, then you just give as much as you can afford. As far as Gentiles are concerned, I don't think it is required since it is not one of the 7 Noahide laws, but is a good thing to do and pleasing to G-d. To me it is logical that the 10% is calculated after taxes, you give 10% of the money that you keep. It is up to you to give it to whichever Charity you want to so you can leave out the drug addicts.