What is the formula Turbotax uses to calculate the Effective tax rate shown on the cover sheet?

Your “effective tax rate” is the average percentage of your taxable income that you owe in federal taxes. In order to calculate this rate, you simply divide your tax liability (what you owe) by your total taxable income.

For more information please check Marginal and Effective Tax Rates

Almost

...the effective tax on my forms it's the calculated tax divided by AGI  (Gross income, not taxable income) (OK...and then, x100 to get a %)

Some folks in prior years have found some other items that affect the "effective" rate, up or down some....but the number is just a handwaving thing to make you feel somewhat better anyhow.

From the TurboTax 2022 in-product help, which I see needs to be updated to refer to the correct Form 1040 and Schedule 1 / 2 / 3 line numbers:

Effective tax rate - year 2022
Effective tax rate is computed based on the ratio of tax to income.  

Income is the adjusted gross income from Form 1040, line 11. However, if there are lump-sum distributions from qualified retirement plans and special averaging or capital gain treatment is selected on Form 4972, the taxable amount of the distributions is added to the adjusted gross income.  Also, if the distributions include net unrealized appreciation in employer's securities, and the option is selected on Form 4972 to include this in the tax calculation, the amount of the net unrealized appreciation is also added to the income.  

Tax is calculated starting with the tax on line 15 of Form 1040, including the alternative minimum tax, but not including the recapture tax from Form 8863 (Education Credits) or the tax from the IRC Section 197(f)(9)(B)(ii) election for an additional tax. It does include the tax from Form 8814 and Form 4972.  The smaller of the nonrefundable credits shown on line 55 of Form 1040 or the remaining tax is then subtracted. The other taxes shown on lines 57 through 61 on Form 1040 are not added to the tax. Finally, the Fuel Tax Credit from Form 4136, the Child Tax Credit from Schedule 8812, the refundable portion of the American Opportunity Credit from Form 8863, the Net Premium Tax Credit from Form 8962, the Health Coverage Tax Credit from Form 8885, the Credit for U.S. Tax Paid to the Virgin Islands and the Earned Income Tax Credit are subtracted. Note that these credits are refundable tax credits.  

The effective tax rate is the tax divided by the income. Because of the refundable credits, the resulting net tax could be negative if the amount of these credits is greater than the tax liability. As a result, a negative effective tax rate is possible.  

If the income as defined above is zero or less, the effective tax rate is set to zero.  

If you transferred information from last year's tax file, the effective tax rate for all prior years columns comes from the information in your 2021 return.

It's way off for me. I have a large self-employment tax that it's not considering. This is what I see:

TurboTax Effective Tax Rate: 10.42% (on page 1 when it prints the return)

But if I divide Total Tax by AGI then I get 23.2% and if I divide Total Tax by Taxable Income then I get 33.5%. Either way, the 10.42% number that Turbo Tax is printing is way off the mark. I wish I was only paying 10%!

My printout states 12.02% Effective Rate: clearly the effective rate displayed is incorrect it had me pay 15%

So what do you trust? Did I overpay???