evilbob
Adventurer
I tried -10 - level - con bonus, but that was complicated and people always forgot it. I changed it to -10 - fort save, which is almost the same thing but easier to remember.
It's only come up once or twice in my campaign, but it certainly seems like most folks give an extra bit of leeway. It's especially frustrating at high levels, when monsters do far more than 10 damage a hit, and having a character who is up at 2 HP is way more dangerous than a character who is down with -2 and no longer a "threat." (Well, you could rule that monsters keep attacking until a player is dead, which would make it more balanced - but really suck for the PCs.)
Just remember that whatever you do, do the same for the monsters/NPCs as well.
It's only come up once or twice in my campaign, but it certainly seems like most folks give an extra bit of leeway. It's especially frustrating at high levels, when monsters do far more than 10 damage a hit, and having a character who is up at 2 HP is way more dangerous than a character who is down with -2 and no longer a "threat." (Well, you could rule that monsters keep attacking until a player is dead, which would make it more balanced - but really suck for the PCs.)
Just remember that whatever you do, do the same for the monsters/NPCs as well.