How did Turbo Tax calculated my tax? I found additional deductions Turbo Tax missed. In order to file 1040X I need to use the same method.

[Carl]

The tax code is extremely complex, and it's impossible to answer your question with a simple one liner. All income is not taxed the same, or at the same rate. How income is taxed, depends on it's source. For example, short term capital gains are taxed differently than long term gains, which is taxed differently than W-2 income, which is taxed differently than self-employe income, and on, and on, and on.
TurboTax calculates your tax liability based on the data you enter and the answers you provide to the questions asked. If TurboTax "missed" a deduction, then most likely it's because you didn't claim it, or you answered a question or made a selection in the program that indicated to the program you did not qualify for the deduction.
If you can prove TurboTax actually made a calculation error, (this does happen a least a few times each year) then it's fairly simple to file a claim and TurboTax will pay any fines and penalties you may incur. Just understand that TurboTax will not under any circumstances pay any actual taxes you may owe as a result of a calculation error.

[SweetieJean]

What type of deductions?

[rjs]

You do not have to use the same method to calculate the tax on an amended return (Form 1040X) as was used on the original return. Sometimes the changes on the amended return require that a different method be used. You determine which method to use to calculate the tax on the amended return the same way that you would determine it on an original return. Line 6 on Form 1040X is asking what method you used to calculate the tax on the amended return, not what method was used on the original return.

You should use TurboTax to prepare your Form 1040X. TurboTax will automatically determine which method to use, calculate the tax, and fill in the method that it used on line 6. If, for some reason, you are not using TurboTax to amend your return, you can find the instructions for determining what tax calculation method to use in the IRS instructions for Form 1040 (not 1040X) for the year that you are amending. It's in the instructions for line 44. For 2016, the instructions for determining what method to use start at the top of the right column on page 41.

[cbinzel]

The other answers are correct: " you do not need to use the same method" , but if you are interested in knowing what method was used, then you want to switch from Step-by-Step, to Forms view (look for selection on the right side of the top blue bar to change). Then look at the "1040 Wks" form. Scroll down to a boxed section titled "Tax Smart Worksheet".  You will see the Tax calculated on Taxable Income on line A, and a check box in one of the 7 possible methods. lines B - H also detail possible additional taxes, but line A and A1-A7 is the method used on your "Taxable Income" (1040 line 10)