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In determining effective tax rate,the numerator is the tax owed and the denominator is the taxable income, correct?

No. At a minimum you would use your AGI - so before the deductions and exemptions. If you want to look at your true tax rate, you would add back in any adjustments you did on the first page, like an HSA deduction, and pre-tax 401(k) contributions, since they aren't included in your W-2 income to start with.

I saw this formula online, plugged the numbers in and was surprised to see it was 31%!

Yes, that's because you're just looking at taxable income - every dollar of which has a tax applied, from 10% on up to 39.6% (plus possible extras). You're not accounting for the non-taxed money, like your 401(k) and HSA contributions.

Also to do a back of the envelope calculation of how different part time jobs might affect my taxes or eliminating the mortgage or increasing charitable donations might affect my taxes is it better to go with the effective tax rate or marginal rate? I may or may not have to pay estimated taxes this year and am trying to figure out the best method to know how much to contribute.

I would suggest using a tax estimator from TurboTax, TaxAct or H&R Block - run different scenarios to see what the impact would have been from this year. That will give you a good estimate of what this year will be, although it won't account for any inflation impacts to brackets, etc.

I've been playing with turbo tax to test out some scenarios but I don't really find it user friendly in the sense that I am looking for something that will provide me with an overview sheet, review or summary sheet of the different scenarios I'm making. All it ever shows is either a refund or tax owed. It's also difficult to delete data to re-enter new data which is frustrating.

Are you using the actual TurboTax software? Or their tax estimator, called taxcaster?

You can try googling for "tax estimator" if you don't like TurboTax's. That said, I haven't used any of them recently, so I don't know user un/friendly they are.

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