AOA, thanks for the response, and my takeaways of this verse break down like this:
1) the context is specifically war time;
2) don't allow yourself to personally profit from any portion of spoils of war;
3) 20% is for the war effort, the state being inherently involved in this;
4) all of the remaining must go to help the needy so that spoils of war aren't construed as an acceptable form of income.
I personally don't see this verse saying anything about the tax rate of a state because of the war time context. I think the tax rate of a country is like its fashion, it's population has to decide how it wants to balance civilization and freedom. I've been watching a bunch of lectures by Milton Friedman and am increasingly convinced that most state services are best left to the private sector, and that we ought to be collectively aiming for higher values rather than higher taxes.