The rule I use is No Save, inflicts 5 HP per caster level, cannot drop HP below target's Hit Dice.
This may seem a lot less than the standard Harm, but it still does more damage than any other cleric spell.
I'd consider using 10 HP per level only with a Will save to half damage.
Heal works the same way (increasing HPs, obviously!).
I have a Greater Heal at 8th level, which heals 10 HP per level - so the cleric has the option of healing a single person (or being) a lot, or using mass heal.
I have no current plans to introduce a Greater Harm, though - if I was going to do that, I may as well use the standard version of Harm for the majority of creatures.