6 Brix is 1.024 on mine. I use Beersmith's Refractometer Tool to calibrate mine.
You can't use the refractometer alone to directly measure the gravity once there is alcohol in there.
Once there is alcohol, you must correct the reading you get by knowing the OG (which you could have gotten with the refractometer). There is a formula that corrects the reading you get post-fermentation, but you must know the OG. BeerSmith does this for you.
Finally, if you want to use both a hydro and a refract, you can back-calculate the OG, and determine the ABV of a finished beer. You can even do this for a store-bought beer (must degas, first). Again, Beersmith's Refractometer Tool does this for you.
Here is everything you need to know about refractometers.