I'm almost certain it's 1 for 1 on the heals. It's split evenly between all targets though, which makes it much easier to not pick up threat on multi-boss fights.
Healing aggro is split evenly between all mobs in combat. It's also 50% as much threat per point as threat from damage.
I can't find the post which specified that it was 50% but here's a dev saying that it is definitely less: