Are you wanting this to act automatically when the mobs hp changes to within the specified % range? Or would you hit a trigger then have it react based on mob HPP?
Would probably be useful to know what you wanted this to do.
Performing an action when a target mob's HP drops below a certain number is the real goal. I know that <if MPPLT="10"> or <if HPPLT="75"> work for MY hit point and MP %, but don't know the code for a target's.