I thought healing by default does 0.5 threat per point? Obviously the case still stands that for each crit or big heal, the tank needs to do big amounts of damage due to the damage reduction the boss has. Waiting till a healer pulls and then doing 2 quick taunt switches should solve the issue for the rest of the fight though.
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.