Health Increase skills and Healing CEs

Any CE that has a % healing increase component depends on the strength of the heal, not the % of the servant's HP. So if MLB'd Maid in Halloween has 75% heal increase, that means a 4000 heal will be healing 7000 HP instead.

For servants with Max HP increase, the skill isn't actually considered as a heal so no boosts will be added from specific % healing increase CEs. If the HP bar does move up with the skill's activation, it's only for the sake of balancing the same % of HP your servant has remaining, relative to their new max HP value.