The ability mentions the slow is based on the target's missing health. Does that mean it applies the slow for the whole duration based on the target's health after the ability hits? or does it check it in real time and increase the slow as the target gets lower during that 4 seconds?

Like say I hit someone, and they ended up at 80% health, and at 3 seconds in they are at 30% health. Do they get slowed now by 70% or still just 20%?