also keep things like: type of racket, string diameters in mind, and open or dense stringpattern..
if you are using a dense stringbed start out 10% lower. with an open stringpattern its a little easier to take, so i would start with a drop of 5%.
of course this al depends on the string you're planning to use.
factors like stringstiffnes, stringdiameter, type of racket play a role here too.