ABV when adding sugar during fermentation

I have been fermenting the Imperial Pale Ale from the Extreme Brewing book. I was wondering how I calculate the ABV after I added 12oz of light brown sugar the 2nd day of fermentation. The OG was 1.054 and is currently 1.021 (at day 5). I'm not bottling for another 2 weeks though.
Should I have taken a reading after adding the sugar?

Math is your friend. This is a case where a reading will probably not tell you what you need to know.

A 1.054 OG for a 5 gallon batch means 54 x 5 = 270 gravity points (GPs). To that, add the contribution of the sugar: 44 points per pound x .75 pounds = 33 GPs.

270 + 33 = 303; 303 / 5 = 60.6 (call it 61). So when calculating your ABV, consider the OG to have been 1.061 (since that's how much sugar ended up in the wort) and go from there.

Now, if you had mixed the sugar in say a quart of water, you would divide the 303 GPs by 5.25 (gallons) instead of 5.

Cheers!

Awesome, thanks for the brew math lesson. Very helpful.