Should be identical to the tax rate on everyone else. Different tax rates for different earnings violates equal treatment under the law. Progressive/redistributive taxation is legalised theft/extortion, and perverts the law and government from administering just laws, into robbing Peter to pay Paul. Personally I think an administrative government of reasonably fair laws applied equally is a lot more ethical than a extorting gangster government which raids one group to bribe another for votes.
I would say that a country with an armed forces capable of defending itself against most global threats, can afford necessary government spending with a tax rate of 15-20%. If some kind of minimum welfare state (e.g. emergency surgery, 6 months unemployment insurance, and basic survival level pensions provided by the state) is desired, then 20-25% is probably more realistic. If you want a bloated military in order to go round gratuitously invading and occupying foreign countries, or a full-blown welfare state, then you are looking at 35-40%, and if you want both than 40-50% is more likely necessary. Anyone think a tripling of taxation is worth it to get cradle-to-grave welfare and unnecessary adventuring abroad by the military-industrial complex?