How do I hang a chair from a concrete ceiling using a single hook?
Your margin of error sounds a bit low - one thing to bear in mind is that while the static force of you in the chair is half the rating of the hook, the peak force will be higher as you "drop" into the chair. If you can get a bigger hook it might be worth it.

If you have a hook on a plate then you'll spread the load across more bolts thus each one won't have to bear the full weight.