You really have to gage how much to do depending on how your fish are "handling the salinity drop.
What I do is remove all the saltwater that I will change out in a day's time, then, over the course of 24 hrs, I add it back in in the form of FW. The next, begin the procedure again. For instance, for my former 20gal tank, I would remove 5 gals a day, and the replace those 5 gals sasy in 1/2 gal increments over the course of the day. The next day, remove another 5 gals, and do this again, until the specfiic gravity is down to 1.009 [not 1.099]. Since I have a refractometer, I also compare the actual salinity which should be 14ppm....14 parts per million of salt in the water. This is the target.
Always observe your fish to see their reaction to the reduction of the salinity. If they get stressed, then, obviously, slow down the procedure. Most stress, however, is more to low pH water or drastic change in temperature, then the actual salinity drop. Be sure to test pH about an hour after dropping the salinity [adding FW] and adjust the pH as needed.