The percentage absorbed in my tests was basically 100%. However, you can only absorb as many HP as the enemy actually has left(whereas you can do more damage than it takes to kill). I could use Foul to do more then 10k damage to a level 3 ladybug, completely obliterating it. If i drained it instead, i might do 1500 damage to it, but would only absorb 30 or 40 since thats all it has. Though i will agree, a stronger version would probly be useful for some solo fights/duties.