The July/August 2013 edition (vol 36/no 4) of Zymurgy magazine has the answer. Sean Terrill's article, "Using a Refractometer" pp. 49-53 details pretty much everything you want to know about refractometers in brewing.
A two-point calibration is recommended:
- Water at 0.0°Bx
- A reference sucrose solution, for example 20.0°Bx
(20.0g table sugar in 80.0g water)
Further, to compensate for the fact that wort is a complex combination of sugars, the refractometer reading should be divided by a wort correction factor (WCF). Take a reading of the same wort sample with a calibrated hydrometer then divide the refractometer reading by the hydrometer reading to generate the WCF. The formula in degrees Brix would be:
X = Refractometer / (Refractometer/Hydrometer)
The WCF will likely range from 1.02 to 1.06. The article implies that this doesn't need to be done on every brew (thus blunting the point of using a refractometer), but establishing the WCF for a particular recipe or style that you brew will be a good ballpark for similar worts.
The article also notes that refractometers with automatic temperature correction (ATC) are calibrated for a specified ambient temperature range, likely 10-30°C (50-86°F); check the device's documentation. Ambient temperatures outside of this range will affect the reading.
It's a good article, full of detail, and I recommend reading it if you use a refractometer.