This spreadsheet lets you compare how much money you'll be able to donate in different careers. It's made for people in the US as other countries have different tax laws. It also ignores city income taxes and assumes a single state income tax system, so it's flawed for comparing careers across different cities and states. But it's a work in progress.
The state taxes I used as default are for California, because it's the most populous state. You can replace them with values for another state.