In the hotkey under the advanced radio button there is some options for GROUP health. If you set this to say 80 it will try to cast that spell when any group member (in this case the leader you are following) is less than 80%.
So a good example would be like:
Single Mob, Priority 0 (because you want to heal as most important thing), delay 4000 (4 second.. or whatever the cooldown of the key is in your game). Set the max range to the range of the spell, ie 10 or whatever. Click add.
Set the key and casting time if necessary, click on advanced, click on group health radio button, and set the edit box to 80.
Click save.