There is no rule, and if you look up verdicts, you'll see that they are all over the place, with some minor injuries drawing huge verdicts or settlements, while some horrific injuries draw comparatively little compensation. The factors that *should* be considered are:
* The degree of impairment or disability--what can the sufferer no longer do?
* The suffer's age--the younger they are, the longer they will have to live with it.
* The medical costs--while not a direct factor in determining pain and suffering, there should be some sort of relationship between the tem
* The plaintiff's lifestyle--what must they give up now?
In practice, sympathy becomes a major factor, with attractive, well-spoken, likeable plaintiffs tending to get more than plaintiffs who are dislikeable. (Makes sense in a way--even though "likeability" has almost nothing to do with running a country, it's always a major factor in presidential elections; why should juries be immune to it?)