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?