They then compute the annual withdrawal available during the specified withdrawal period. The accounts deplete at the end of the period you specify. You may want to look at assets in today's dollars. Some programs do include an inflation rate to allow a translation of future dollars to 1988 dollars. This is particularly necessary for the post retirement years when taxable income may fluctuate considerably, especially if you are constrained by the variable minimum required distributions.
Some programs allow an annual indexing of the tax rates.
In summary, the Strong Fund Roth IRA Calculator is a good start towards the consideration of a Roth Conversion. You may want to consider using higher end programs to evaluate a range of scenarios that reflect your retirement plans. The author Dr. Gobind Daryanani is the founder of DQI Inc., a company providing Roth Conversion Consulting Services. DQI also offers a Roth Conversion Service , which uses the best available software programs to evaluate a wide range of scenarios for clients.