Cutting down a percentage value of Life

Well my spell weakens an enemy unit by a 25% of the enemies' current life.
Any ideas how to do that with triggers? (This might be very easy but I just cant think at the moment ://// )

arithmetic - unit's current life (unit - property) x .25

use conversion real to integer if the above doesn't work.

btw, the trigger to set a units life is unit-set life

piro i think u meen 0.75, then the unit will be weakened by 25%