The problem with hydrometers is two fold, some do not measure low enough and they are prone to inacurracies. The link provided above is also not the best suggestion. If using a hydrometer, the target goal should be 1.009. If the salinity is allowed to be to high (above 16 ppt) the treatment will fail. At best, all you would do is slow down the parasite, not kill it. Do not try to convert ppt to
SG, the refractometer is not affected by temp, the hydrometer is.
Cheers
Steve