Question on Medipac use

[GingerLotus]

Reading the description for it

NOTE: Because the medical database is programed only for
humans or pre-disaster living creatures, there is a chance
(5% for each hit point healed above the creature’s maximum)
that the medipac will remove one random mutation or defect when used on a mutant, manimal, or plantient.

Do most GMs roll the 3d8 for total healing, then heal up to the creatures max, then use the left over to figure the % chance of removal? Say for example PC has 12 hp total, the 3d8 roll is 18 healed, PC is wounded and at 3hp currently, is the % chance 30% for 6pts over the full max or 45% for 9 of the 18 points used to heal?

Just wondering on how other GMs handle this

[GnomeBoy]

Certainly sounds like a 45% chance to me. The "unused" points multiplied by 5 is the chance...