I pulled this off of another board. This equation agrees with the calculator in Promash (or at least close enough).
Alcohol by Weight(abw) = (OG-FG) * 105
Alcohol by Volume(abv) = abw * 1.25
So for your beer, take 1.045 and subtract your gravity reading, multiply by 105 and then multiply the entire result by 1.25 to get %ABV (most everyone means %ABV when they talk about % alcohol.