Trigger question

How would go about subtracting health from a unit? All i can find is modify via percentage and modify via amount, but I want it like "subtract 500 HP from units current HP" or something. Does it involve variables? In that case how do you store a units current HP as a variable? thx ahead of time

You could use something like the triggs I used in a more complex example :) anyway it picks a unit and sets the life of the picked unit minus whatever value u need, it's done using the Set Unit Life Value not Percentage trigger. Here is a cropped image: