Ah, OK. I'm not sure which screen specifically you're talking about, but I do find some notes in the program that discuss how we do the effective tax rate calculation (in at least one location in the program anyway). It looks like we use Adjusted Gross Income as the income for this calculation (line 37 of Form 1040). However, the tax being used is kind of a hodge-podge of some taxes but not others. For example, we subtract the earned income credit that shows up in the Payments section of Form 1040 -- so you could wind up with negative tax and a negative effective tax rate...on the other hand we don't include alternative minimum tax, so if you pay significant AMT the effective rate might look a little lower than what you're expecting.
Since it's not a specific term, and just a tool for getting a rough idea of how much tax you're paying "on average" on your income, you can get a reasonable estimate by just dividing the line 61 total tax by your line 37 AGI.
