Some guy dusted off is algebra book and came up with a formula that IMO is fairly accurate, albeit somewhat annoying to remember. I think it's more accurate than the (OG - FG) x 131.25. The only caviat is that you must use the whole number of the gravity reading (i.e. 1.056 is just 56 in the formula). Anyway the formula is below:

((832+OG) * (832+FG) * (OG-FG)) / 5,500,000 = ABV

For instance:

OG = 1.060

FG = 1.010

832+60 * 832+10 * 50 / 5,500,000

892*842*50 / 5,500,000

37,553,200 / 5,500,000 = 6.8% ABV