I have one of those hydrometers. I will tell you that it said my salinity was 1.022 and when I checked it with a refractometer it was 1.028. I know refractometers can be expensive, so I will give you a trick I did until I bought a refractometer. Take some water to the LFS
and have them test you salinity. Adjust your water according to what they test. When the water test where you want it, then test with your hydrometer and mark with a sharpie. For example, I got my water down to 1.024, which on my deep six hydrometer was 1.017. It got me by for a few months.