*sigh* No, you're only half correct. Healing monsters is based on INT/MATK, and
so is healing party members. Ask any STR cleric... They can't heal themselves or their party members for more than 20 HP or so.
The in-game description is misleading. It says 300%, but it doesn't say 300%
of what. The 300% is a multiplier to some unknown, hidden INT/MATK formula that the game calculates. LUK is also possibly involved, but only slightly, if at all.
Sources:
Best approximation for heal damage formula.
Afrobeans's HP cleric:
here,
here,
here. Unfortunately, the main thread about it got deleted because it was too old...