What if it isn't fully "Quitting"? Her income will be reduced by 80% and made up by the paramours income.

That's the circumstance the appellate courts contemplated in coming up with the rule I mentioned. The judge will do one of two things in that circumstance: either add in the new spouse's income or attribute the old income to the person who just "reduced" her income.

Do you have a case reference for that appellate ruling? So it sounds like moving in with the paramour to significantly reduce her income would be at least admissible in a modification hearing?

I won't do your research for you. I get paid to do that. But if you want to go to the law library, you are searching under the term "voluntary underemployment."