To be honest, I take brix readings and just multiply by 4 to get my SG readings.
10 brix on the refractometer ends up being 1.040 SG. There is a small variation when you get up into high gravities like lets say a brix reading of 22, which ends up being 1.092 and not 1.088.
I've never calibrated mine...tried regular tap water and got a reading somewhere between 0 and 1.
I did verify using a hydrometer when I first got the refrac. the hydrometer was reading 1.042 and the refrac was reading somewhere between 10 and 11 brix. close enough for me.
when I pressure ferment, I take SG readings with the refrac as well. I just tip the keg on it's side, take a small glass, put it over the gas out post, use a sanitized end of a pen to push the poppet in, get a small spray of beer in the glass, let the foam dissipate, and take a couple of drops for the refrac. works quite well actually....would definitely make too big of a mess trying to get enough for a hydrometer.