Yes a refractometer needs to be calibrated and some hydrometers are very accurate. But for hyposalinity treatment it needs to be right on. Do not do a massive water change, personally I would not do anythng greater than a 20 %, but rather several smaller ones over the course of the next 24 - 48 hours IMO. The fish are already shocked by the ammonia, pH and SG levels it is not going to take much to put them over the edge. Do you make up your own salt water or do you buy from the LFS ?