MATH!... ok but you guessed that.
step one. Take the OG. Step 2 take how much sugar you add. step 3 take the FG and run it through the math.
Basically the og describs the density, but we call it in points, so a 1.040 has '40 points if that is 1 gallon, then it has 40 points. If it has 5 gallons, then it is 200 points (40*5).
Fermentables (sugar, malt, etc) have a standard 'point per pound gallon' value. Table (brown and something else) have a pppg of 45. So 1 pound of sugar in 1 gallon of water shoudl be 1.045 SG.
Anyhow. If you had an OG of 1.065 on this DF 120 min clone (yes I'm making up numbers now) and added 2 lb of sugar with a start volume of 5 gallons, your original points woudl be 5*65=325 and you've added 90 points (45*2) for a total of 415 points/5gallons to get the 'assumed OG' gets you 83 or 1.083 If the FG comes in at 1.020 then you can figure the ABV from there.
Anyhow give me the numbers and I'll happily work the math for you.
BTW, where abouts in Manchester NH? I used to live in the area.