If you feel like doing some experimentation to get things very accuate, check out Stan's Beer and Wine Page
for some ideas on calibration of your hydrometer. He has some good basic information about the process. If all checks out then calibrate the refractometer to your hydrometer.
I believe that the brix scale on a refractometer is defined for sucrose solutions, but has some inherent errors with more complex sugars as they don't have the same refractive index. I don't have any idea of the magnitude of the error though.