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Your calculations make sense, Barry, and I too have worked out a flat-tax system, based on the tax system of Ancient Athens. According to this system of the Athenians, introduced prior to Pericles, however, there were tax groups, five I believe.
The lowest paid no tax at all, but the higher the tax category the higher the set amount of tax that was due each year,
An individual paid annually the amount of tax which their category prescribed, regardless how much they earned.
They would only pay more if they moved on into the higher category, which depended on the value of their possessions, ie land, ships, workshops, houses...