They gain before they lose correct?
I would only target mark quanta if they had 14 or more quanta already. Although counting the cost of Debilitate I would probably wait till 18. Obviously this is only relevant when there is not a more tempting target.
(13 + 12)/2 round up is 13
(14 + 12)/2 round up is 13
(18 + 12)/2 round up is 15