Earnings is defined as all income irrespective of source
Brackets defined by Earnings less or equal to Bracket Limit, and a Bracket Tax Rate. Thus, tax due is simply Earnings * Bracket Tax Rate
The Bracket Tax Rates can be arrived by analyzing the current taxation data and figuring out the actual rate paid (after all deductions) by those with that level of earnings.
Keeping in line with the President's policy, the rates can be set such that X percentile of folks in the brackets, and with earnings less than 250K (or whatever the limit), do not see any increase in taxes -- it would be even better if these folks see a reduction in taxes (also easy to sell to the public) and economy boost.
This will reduce the tax shenanigans of tax dodges etc.