[Spivak]

Without any sort of metric which defines what a successful tax policy means then reasonable probably means, "is sufficient to pay for the services the people demand."

Why is a percentage reasonable? Why not a fixed tax? Why have tax brackets at all as opposed to a smooth curve? Why should taxes scale proportional to income and not amount of services consumed or potentially thousands of other variables? Why should a pacifist pay for the military? Why should a pedestrian pay for the roads? Is it fair to use taxes to manipulate the incentives of consumers to they consume more or less of specific products (like corn and tobacco respectively)? Is is ethical to use taxes (or lack thereof) to steal businesses away from other states or countries? Should people who work in certain industries be taxed more or less? How about different tax rates based on age, race, sex, or gender as there's probably data to support different risk factors for different segments of the population. Should people who have been on welfare be taxed more to compensate? Should the tax rate be negative for certain income levels?

I have absolutely no doubt that you can answer all these questions relative to your own personal definition of reasonableness but there's no chance in hell any sufficiently large group will agree on any or all of these.