I have a digital refractometer, not the brand that the OP referenced but similar.
I've found it works very well and, after comparing a reading from it and a hydrometer for several batches, I've found that the two are always within a gravity point or two of each other.
I don't use the refractometer for the FG reading due to the alcohol.
Which model do you have? There's one from Hanna Instruments also on amazon.
Also Sean Terrill's site does a great job at calculating final gravity using a refractometer.
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