easy rule of thumb
For an active person (working out 3-5 days a week) I have found that the best way to determine my protein intake is simply eating 1 gram of protein per lb of lean body mass.
That means you take your body-fat percentage * your weight to determine your lean body mass. I have been on a cut since about November, and doing this has allowed me to drop fat while maintaining muscle tone.
It is an easy rule of thumb that I have seen recommended on a number of body building and weight loss threads.