It's not doing the calculation twice. It automatically fills in the business percentage use based on the numbers of rental/personal days. It just does the calculation once, but it applies the 50% ratio to the cost basis instead of using the full cost basis and then applying the 50% after calculating full depreciation.
I'm not sure which method is correct. The worksheet is pub 527 is not much help. It just says:
"Enter the rental portion of depreciation of the dwelling unit."
But it doesn't explain how to calculate the rental portion of depreciation. Whether to apply the rental portion to the cost basis and then calculate as normal (using the formula method, not the tables) as TurboTax does. Or whether to calculate the deprecation as normal with full cost basis and then apply the rental portion to the final calculation.
