There are several formulae that estimate alcohol content from RI and SG but none of them are particualrly accurate. The method is used by breweries for quick checks but the catch is that the refractometer has been calcibrated agains a densitometer and the calibration only applies to the particular beer or, at best, family of beers. MEBAK has formulas for schankbier and vollbier. Those "families" are a little broad. Neither gives a very good results for the beers I brew and I've never tried them (or, for that matter, the distillation method) for low alcohol beers. Even at the 4 -6 % level densitometer reading can be "noisy"