You should click on page 1 at the bottom of this discussion and review all the posts from March and April 2023. Back then, a turbotax customer service rep told me that they divided Total Tax by AGI (adjusted gross income) instead instead of Taxable Income to calculate the effective rate. That didn't make sense to me because the IRS Tax Computation Worksheet (1040 Instructions, Pg 77) uses Line 15, Taxable Income, not AGI. I tried using AGI on figures from my past 5 years returns, and that calculated pretty close to what turbotax was showing (exactly the same in some years). Turbotax should be using Taxable Income in the effective tax rate calculation, the way the IRS does.
