The Crimson Binome
Hero
It's an objective thing, so you can measure it. Are you using the proportional HP model? Great, so 5/100 is the same as 1/20. Gather a field of statistically identical individuals, and find a weapon (a cudgel of specific size, swung with a standardized force) such that it takes exactly 20 hits before one falls unconscious.Yeah, but, "How hurt are you" is a pretty vague question. For example, how hurt is this character? He's lost 95% of his HP, but, what does that actually mean?
Then, go to the next individual in line, and hit that person exactly nineteen times. The character in question is exactly as injured as this individual.
(Also, just FYI, 3.0 used 1 hp per level per day for rest, or 1.5 for full bed rest; and 3.5 used 1 per level per day, with 2.0 for full bed rest. The upshot was that, no matter how many maximum hit points you had, you recovered from zero to full in about one day per average number of hit points per level - or about a week.)