I would be interested in seeing what values of ax+b (a being the rate and b being the standard [+ perhaps only] deduction) would work out to be income neutral to our current convoluted algorithm.
Define "income"
Most flat tax proposals I've seen exempt the first $x. So it ends up that the people hurt the most by common flat tax proposals are the middle and lower-middle class. Upper-middle is basically the same effective tax rate and beyond that pays substantially less. The only time I've seen the upper brackets pay a higher ETR under a flat tax is if it includes all unearned income which is almost never the case.