If this retirement planning were for me, I would add taxes as a budgetary item. (Along with all things in life, if the costs increase, which they may very well do, then planning and tactics may need to change. Hopefully I have built in a little fluff to account for slight increases.) For me, much of my retirement income will come from money that has already been taxes, so this is really a worst case scenario.
For my budget:
Assuming zero tax advantaged or after tax accounts (or only Traditional 401k/IRA/pension income), a $20,000 per year income for a single person with no additional deductions equates to a taxable income of $9,700 in 2015 ($4,000 personal exemption and $6,300 standard deduction). This equates to a 10% rate on $9,225 and a 15% tax rate on $475, or $993.75 in taxes.
The great thing about these bottom two tax brackets is that dividend and capital gains income is currently taxed at 0%, so the ~$6,000 in income that I expect from these are tax free as well. Because of my situation, I would assume taxes of about $500 in my budget (this includes a small amount for state taxes).