Magic Power Attack


It says that for every 2 power points spent when casting spells that affect only creatures of a limited number of hit dice, that number is increased by +1.

Could you determine the hit die of your intended opponent? or is this basicaly a wager you make to see if you reach the hit die? I you don't beat the hit die of the opponent, are all the power points lost?
I believe you have to gamble. If you didn't spend enough PPs, all the PPs you spent are lost.