fermentation/gravity

My algrain 5 gal batch has been fermenting for 12 days and got so vigorous I had to put on a blowoff tube. My O/G was 1.051. I just took another reading and its at 1.02 and kinda carbonated. I know I need to do another 1 or 2 readings to see if its done and ready to go to the secondary, but how do you determine the ABV on that reading?

 

ABV is 4.1% according to beersmith.

 

Usually hydrometers have a few scales on them. One is Gravity, one is percent alcohol. 1.051 is very close to 7% ABV, 1.020 is very close to 2.75% ABV. To find out the actual ABV subtract 2.75 from 7 = 4.25% ABV (Make sure to adjust your gravity readings as necessary for the temperature of your sample)

There's another simple math way to check as well but I don't remember it. I think in John Palmers online edition of "How To Brew" he explains it.