I had a Yorick top come to my midlane in a game not long ago. He was level 16, I was 15 and doing okay as Morg vs a Zed (we mostly traded 1 for 1). In the fight he ignited me (he had ignite which was weird enough) and it did it's 455 true damage as expected... except, I had no extra HP items, just base HP at the time (other than dorans). It did 25% on my HP. This is the first time I realised how much it really does.
At level 1 it will do 13-14% of my HP, which is more reasonable. It scales better than most champs base HP so if you don't build bonus HP (like a lot of midlaners, ADCs or Enchanters generally don't) it gets stronger as the game goes on.
I would think Ignite should be an early game kill secure summoner spell while late it should be used more to stop healing due to it's grievous wounds but at the moment it just does a lot of damage.