The Death Kiss, in Volo's Guide to Monsters, has an ability that says
On a failed save, the target takes 4d10 (22) lightning damage, and the Death Kiss regains half as many hit points.
Yesterday, the Death Kiss used this on a Barbarian, who resisted the damage, an only took 11 points of lightning damage. I ruled that the Death Kiss still healed for 11 HP, but the players mostly disagreed, and claimed it should have healed for half the damage it caused, so only 5 HP (rounded down).
On abilities like this, do you account for damage modifiers belonging to the target or just the original value?