The refractometer is for determining the amount of sugar in fruit and juice before fermentation. Once fermentation begins you can no longer use it without an excel spreadsheet that would convert you readings over to sg.
I did cross reference the Brix to SG on the chart and it does not coincide. With a Brix reading of 10 I should have a SG 1.030, however my actual SG is under 1 which makes me think the brix reading should be zero as well.
Lol Doug you got in before me this time! As Doug said the refractometer is for fruit before fermenting. Using it afterwards you have to use a complicated calculated formula. There are spread sheets out there using Excel but you have to enter all your brick readings from the beginning. Hydrometers are most accurate when properly used.