I was able to duplicate TurboTax's Effective Tax Rate (ETR) using the following formula:
ETR = (Total Tax - Other taxes including self-employment tax) ÷ AGI
Why it is calculated as above (rather than ETR = Total Tax ÷ Taxable Income) remains a mystery to me, but I am not a tax expert. In my opinion, excluding self-employment tax from the calculation for people with self-employment income makes ETR a meaningless (and misleading) number.
