Ok, so let's say the healthbar would be maximum 150 pixels in width.
if the maximum health = 500 and the current health is 250 the bar would be (250/500)*150 = 75 pixels, which is half, so correct..
so the bar should be set to (CurrentHealth/MaximumHealth)*HealthbarMaximumwidth..
If your enemies have different maximumhealth and you would like them all to show their health relative to one another, setting the MaximumHealth to the MaximumHealth of the enemy with the highest MaximumHealth should do the trick..