You have a 16th level fighter named Bob. He has just fought in the Battle of Nu-Nan, defeating the Demonlord and all his minions. Of the 160 hit points that Bob had going into the battle, he now only has 80 left. The cleric comes up to Bob and says "Wow that is a SERIOUS wound, let me heal you!" You of course, say yes and prepare to receive a bountiful 10 hit points back. Bob looks at the cleric, "I thought you said you'd cure a Serious Wound?" The cleric shamefully ducks his head and mumbles something about gods and their gifts.

I have always had problem with is magic healing in the game of D&D. It is great at low level, when a Cure Light Wounds can heal you completely but as you get higher in level, the spell starts to seem weaker and weaker. Even in potion form, the Potion of Extra Healing can only heal a max of 27 hit points, the equivalent of a Cure Critical Wound. To a character with more than 50 hit points, 27 points of healing just do not heal a critical wound.

Now change the spell into a percentage of original hit points and it takes on new meaning. The Cure Light Wounds will heal 25% of recipient's original hit points, the Cure Serious will heal 50% of Original, and the Cure Critical heals 75%. The Heal spell stays the same, including all the disease and blindness healing, because it is basically a heal 100%.

Now in the example above, the cleric heals the fighter for 80 hit points of damage (160 original * 50% would be 80 hit points). The flip side to this is that the spell Cure Light Wounds will no longer save your bacon at low levels. You try to Cure Light Wounds the Mage with 4 hp, and he gets 1 point back. To off set this, set a minimum heal of 4 points for the Light, 9 for the Serious and 18 for the Critical. So no matter what the percent of original would be, the Cure Light Wounds spell will always heal minimum 4 hit points of damage. Healing potions should work the same way with a standard healing as a 25% potion and an Extra Healing as a 75% or 3X25% potion.

The other thing to think about is the evil cleric. The Cause Light Wounds just became a much more dangerous spell. The reverse of the spell would work on the same percentage, making that evil cleric a creature of great power and damage. This gives the cleric a more offensive spell in their list other than Flame Strike and Spiritual Hammer.