When I'm GM, I add up damage to monsters.
When I play, I subtract damage from my hit points.
Why? Because when I'm GM, I generally am not so worried about exactly how many hit points each critter has remaining - what I'm most concerned about is if they are still standing. When I'm playing, what I'm most concerned about is how much longer I'll still be standing, so knowing how many hit points I have left is more important.
When I'm GM, I add up damage to monsters.
When I play, I subtract damage from my hit points.
Why? Because when I'm GM, I generally am not so worried about exactly how many hit points each critter has remaining - what I'm most concerned about is if they are still standing. When I'm playing, what I'm most concerned about is how much longer I'll still be standing, so knowing how many hit points I have left is more important.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.