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.
