It is actually pretty easy to figure out the gravity increase from the sugar addition. Sugar is pretty much 100% fermentable. It will add 46 pppg. Since you added 2 pounds, you added 92 gravity points. You divide that by the volume of wort in the fermenter. Assuming you had 5 gallons, this would give you 18.4 points or an increase of 0.0184 over the OG you had at the time you first pitched your yeast. Just use that revised OG when you compute attenuation and %ABV figures.
Wayne
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