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not sure where you got that from...no line 26 in 2019 tax forms....you meant 16?

Probably the wrong lines anyhow 

It's a handwaving number anyhow, and you may only get an accurate accounting of "exactly" what is used by calling Customer Support during the week (probably not open weekends).  A few of us have been puzzling a bit over this exact calculation for a few years now...but mostly it's a "shrug".  For a few low income folks with EIC, and child tax credits, it can even be a negative number.

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Try AGI / tax.     (For 2019 forms)

Thus:  line 16 divided by line 8b  (this matches one of my test tax returns, but not others)

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Your Interpretation of "effective" may vary from TTX's interpretation....but AGI is before considering any Deductions and certain credits, and thus is closer to your total income.  And, I might argue that "effective" should include tax-exempt $$ too.

a) ...i.e.  line 16 divided by ( line 8b plus line 2a) 

or maybe before child tax credits too and SE taxes too..so maybe:

b) Line12b divided by ( line 8b plus line 2a) 

or maybe some folks might consider the subtraction of line 8a shouldn't be used, so:

c) Line12b divided by ( line 7b plus line 2a)

and perhaps

d)  ... any non-taxable portions of a Social Security distribution maybe could be added to AGI too...after all, it is still income you received, but did not add into AGI.