If I shared a marketplace policy with my parents and they paid the whole amount and received the whole tax credit, is my percentage 0?

If your parents are choosing to claim 0% then yes, your percentage would be 0.  Basically, you and your parents can agree on how much you both would claim.  Many times parents will claim the full 100%, but you do need to talk to them to be sure that is what they are doing. 

Also, if you are a dependent on their return, you would not enter the 1095-A on your return.