You need to figure out what results you're looking for before you start adding lead. Adding lead just for the sake of adding lead is a waste of time, in my opinion.
Do you want to move the sweet spot? Increase the stability in a certain direction? Increas the mass without changing the balance? Increase the swingweight?
What are you trying to do? My opinion is that for the majority of tennis players, adding lead never leads to any real positive results. It can become as distracting as this racket business. Pick a solid racket you're comfortable with and concentrate on your game, that's my advice.
Anyway, if you still plan to experiment, check out this great website-->
http://www.sandiars.com/reference/racquet-customization.htm
A little bit of lead goes a long way.