The CD/Download TurboTax software also has a What-If Worksheet that makes it very fast and easy to do the kind of experimenting that you are talking about, and it doesn't affect your actual tax return. It works something like a spreadsheet. TurboTax will initially fill in the information from your tax return. Then you can change the 401(k) distribution amount and immediately see what happens to the taxable Social Security and the amount of tax.
