Nonei
Explorer
The houserule that currently use is that we follow RAW up to -9. At -10, you are mortally wounded. If you are bleeding, you continue to bleed until you make your 10% check. You can be healed normally, but you are unconcious for 5 minutes regardless of improved health, and you lose 100xp/level. If you pass -20 you're dead.
So I have been thinking about this... I think my suggested houserule would include "mortally wounded" (with the associated negatives as listed in my quoted post above) with death occurring at -20.
If a spell or ability says it puts something at -10 or kills it (i.e. save vs. death), then the character would be automatically at "mortally wounded" (-10) and must then make a second save to avoid dying.
When an attack or action physically takes a PC below 0, they must make an immediate fort save vs. DC of 5 + half the negative damage. If they fail, they take full damage or go to -10, whichever is higher. If they make it, they take only half of the portion of the damage that puts them in negative (round up) or are at -9, whichever is higher. If they make it by 10 or more, they take half the negative damage or -5, whichever is higher.
Then the PC's initiative is moved to just prior to when they fell. Each round they must make a fort save vs. their current negative hp following mostly Herzog's suggestions above with a couple differences (my changes are in italics).
1. Make a Fortitude save equal to your neg hp.
2. If you fail by 10 or more, you automatically move one step closer to death (i.e. if you were dying, you are now mortally wounded (-10); if you were mortally wounded, you are now dead (-20))
3. If you succeed by 10 or more, you are stable.
4. Normal methods of stabilizing (cure spells, Heal checks) work as normal.
5. Otherwise, you lose 1 hp.
6. On your next turn, if you are not stable, you are still dying. (see 1.)
1 is automatic failure, 20 is automatic success.
Does that seem reasonable?