It's based on the MHz * 1.18 calculation to give AMD XP processors a number similar to Pentium 4s. Divide your PR by 1.18 and you'll have something very close to your clock speed.
It's either PR or Performance Rating, btw. Saying "PR Rating" is redundant, like saying "PIN number" or "ATM machine". A common mistake but technically incorrect, if you're nerdy enough to care, like me.