I calculate hit points remaining. In 3e, the math worked better if you did damage taken,* so I used to run things that way, but now I'm going back to current HP.
3e didn't admit this up front, but if you look at it, the way constitution and temporarily gained hit points work only makes sense under a "damage taken" paradigm. 3e actually expected you to do a small amount of mental juggling in order to accomplish a "damage taken" style outcome, while writing down "hit points remaining." It was better to cut to the chase and count damage taken.