I like your idea of about 6% of GDP. Any "charitable institution" is supposed to be able to keep its overhead below 7% if its total costs and give the rest of the money to the cause, otherwise they are wasting your donation. I see GDP in the same light and so the same should be true of the government.
Only problem is the one mentioned in the quote above. Because the original income tax was implemented during the Civil War, what happens when a future "emergency" whether real or a figment of RAT immagination requires the government to have more funds? This concerns me. It seems like some sort of constitutional amendment might be required to clearly outline the limitations of such an emergency levying of taxes. That would probably be quite difficult to manage.
BTW - My last attempt - any chance of getting my screen name changed?