FAFSA - Tax information after marital change

My parents were divorced this year (2018) and the information that the FAFSA requires is from 2016. I began to fill out the tax information for my mother because I primarily live with her. It tells me to subtract her income from my father’s (which makes her AGI a very small amount. Not sure if this will raise red flags or something) but then it asks for her exemptions and income tax. All of her income is from her self-employed business so we know the exact amount she made. How do we find her income tax based on this and does self-employment tax have anything to do with it? I can’t find any answers on the FAFSA help pages or FAQ.
I’m a senior in high school filling out the FAFSA for the first time, by the way.

Thanks guys.

What FAFSA are you filling out? If you’re now a high school senior graduating from high school next spring, the first FAFSA you should be filling out is the 2019-2020 FAFSA, which isn’t available yet (comes out on October 1) and will use 2017 income.

Well…

It appears I did infact fill out the 2018-2019 nearly to completion. I was under the impression that you could start it beforehand and submissions opened on the 1st of October. Dammit.
Anyway, I think my question will still be relevant on the 2019-2020 application considering the divorce happened in 2018 and we will be using 2017 tax information which was still filed jointly.

Yes, you have to figure out what the 2017 taxes would have been if she had filed alone (probably as Head of Household). Take a blank 2017 tax form and fill it out with only her income and deductions. You will have to enter it manually on the fafsa, not using the DRT. It is very likely she’ll be required to fill out verification forms so watch your emails from the schools you are applying to.