[Randy Holmes-Farley]

Well, close to 3.5%,

FWIW, a refractometer that reads fro 0 to 28%, you are going to have very poor resolution in the range of 3.5%.

The link above says the RHS28 has a resolution of 0.2. Assuming you can get that in practice, then you'll be seeing 35 +/- 2 ppt, and that's not especially precise.

It may be adequate, but not optimal.

Be sure to use a seawater standard to calibrate this refractometer as it is a salt refractometer, not a seawater refractometer. So 3.5% is not exactly 35 ppt as made, even if made perfectly.