you might want to buy the Protein Power book.

It has an elaborate method of figuring out your % body fat (based on your wrist and waist measurements), and from that, your Lean Body Mass (everything but the fat, I think...), and from that, how much protein - minimum - you need to sustain that lean body mass. You can adjust the amt. based on your level of activity and age, etc.

this system just gives you a minimum protein intake. I think you are allowed to eat more than that, if you are hungry.