From 20% bonus HP to 15%. At face value, this makes me just choose champions who don't build Cinderhulk or to use Cinderhulk alternatives if possible. For example, I'd go Warrior Skarner, Bloodrazor Jax etc. The limit for me to not avoid Cinderhulk is probably 17% or so.
Why not just make it scale on levels? 12% level 6, 16% level 11, 20% level 16. I don't think this would make me abandon Cinderhulk because at least late game would still be there but it would definitely be quite weaker early.