Your best option is to create a new return in the current year version of TurboTax to estimate what your taxes would be. This allows you to make changes to your assumptions until you have the result you want. It's unlikely the tax code will change sufficiently to make this estimate unreliable. The IRS does not charge an overpayment penalty. But they also don't pay interest on the excess tax when you file next year with a refund due.
We recommend TurboTax for Desktop for this purpose because you can view all forms and not just Form 1040. Also, you can prepare as many returns as you like in the desktop version, including any states that you already purchased, for free as long as you don't actually file the return. Note that you won't be able to use this test return to file next year because TurboTax software is specific for the tax year of filing.
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