the amount you changout the the water has alot to do with your skill level compounded there after by what your keeping. both fish and corals have different input needs, and the latter resulting outputs!
higher water changes are needed if you have a hefty bio-load, less with a balanced reef, even less, like 10% a month zone for an incredibly stable evironment!
2 questions then.....did your
LFS ask what you kept & what and how often you feed? and does your replacment salt contain
trace elements both minor and major>?
answering those 2 question and your one step closer to answering your question!
if you do have a balanced sythetic mix it can help ionicly and osmoticly to change out with that percentage, although as the others have mentioned its no use flying blind with the quickest changing medium in the world! "test 2 assess" that way you know if your system need manual dosed inputs!
