Formula for % ABV

What formula does the software use for alcohol percent by volume?  Putting in sample data gives results closer to the old homebrewers fudge of ABV = (og – fg) * 131.25 than the more accurate formula established empirically by Balling, but not exactly matching either.  I think the software should use Balling's formula.

See
http://www.brewersfriend.com/2011/06/16/alcohol-by-volume-calculator-updated/

or better yet, DeClerk, vol. II, Chapter XI.