I was covered under my parents' health insurance last year but I'm filing independently, how do I figure out the percentages to claim on my tax return?

No, you do not claim any medical expense on your tax return.  You only claim what you paid out of pocket on your tax return.  

However, when you get to the Health Insurance section, you will indicate that you were covered.  And you only have to enter information about your health policy if the policy is through a government Marketplace (ACA, Obamacare).  If the policy does not send a 1095-A to your parent(s), answer NO to that question and you will be finished with the Health Insurance section.