The Online web versions open in early December. You can set up more than 1 account to do practice returns or just use the same account. But be sure not to file any of them except for the real return when you are done.
If you buy the CD/Download Desktop program you can do unlimited returns so you can test different amounts. The desktop program is available now here,
The CD/Download program has many advantages over the Online version. But you need a full Windows or Mac computer to install it.
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.