HP Increase/Decrease by % Value?

Hi, I'm wondering if there is a way to set an HP increase or decrease on an item by a % Value rather than a set value. I have a plugin that increases HP per level but I want to create an item set that will decrease HP by 50%. Their Total HP could be anywhere from 20-40 (10-20 hearts) depending on their level in the server so a set value does not work well here.