Your final tax rate can be much more complicated than what is on the tax tables. The tax tables assume that you have no long term capital gains or pass through income any other income taxed at a different rate.
TurboTax provides a refund calculator here although the math is not out there for you to see and I assume the formula is what you want in order to program your spread sheet.
If you break out each type of your income separately and then calculate the tax rate for each type of income at it's proper rate and total it in the cell that has what you will owe that should get you where you're going.
@roscoedillon
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