Say boss does 10 damage per turn. If you heal 8 points per turn, that means boss effectively does 2 damage per turn, and your effective HP is 5x what it would have been without healing. Heal 1 more per turn, for 9 total and your EHP is 10x. Heal 10 per turn and your EHP is infinity. Heal 11 per turn and your EHP is still infinity, no change. So that 10th point of healing was worth waaaay more than the 9th or the 11th.
Say boss does 10 damage per turn. If you heal 8 points per turn, that means boss effectively does 2 damage per turn, and your effective HP is 5x what it would have been without healing. Heal 1 more per turn, for 9 total and your EHP is 10x. Heal 10 per turn and your EHP is infinity. Heal 11 per turn and your EHP is still infinity, no change. So that 10th point of healing was worth waaaay more than the 9th or the 11th.