Your parents would use just their income. It is just your income you would use on your 8962. It probably would work best if you allocate 1.00 to you and 0.00 to them, if they received APTC, since your % of FPL is lower than theirs.
You are going to need to coordinate with your parents. This will be a decimal number representing your share, and you'll use the 1095-A figures muktiplied by that decimal. In this case, you and your parents both fill out Form 8962 as part of your tax filings, and you each use Part IV to name the other and to choose an allocation. This is legal, and young people up to age 26 can be covered on parents' plan even if not tax dependents. If you were not able to be claimed as a dependent by your parents, then the ACA marketplace plan that covered you all was covering two separate tax households. The 8962 would then reconcile the amount of tax credit they received. If you were a dependent, you would have a filing requirement because your earned income was over $6300. In that case, their household income includes their income, plus your income if you had a filing requirement. If you were able to be claimed as a dependent of your parents, then they should be filling out Form 8962 and you would not.
