How do you put value on determining pain and suffering?

There's no formula anyone can give you, and in one very real sense it's an art:  pain and suffering, for a particular injury, is worth what a skilled attorney can convince a jury that it's worth, within reason.

Some of the things that make a difference, in the value of a case, include permanent changes in the injured person's life, in the wider sense.  Losing the ability to run, as a result of a leg injury, is worth more if the plaintiff was an athlete or hiker, compared to someone whose passion is tournament chess.