all you have to do is make a game out of it!
Pick a number like, 80.. Slow when mob gets to that %.. Then take note of what % it's at when it lands, if it seemed worth it (how many hp did you save the tank in the time it was slowed.. Maths.. Compare to mana cost and rate of heals etc) the try to +/- the 80 to like 85... See how high you can get it before you get Agro...
Then try to break that record.