the way it was explained to me: adding sugar will increase your OG, but it is 100% fermentable, so you are actually adding alcohol with a gravity lower than water, reducing your FG
not getting it to work in online calculators, but if you have Beersmith, create a new recipe and add 1lb of Sugar, Table (Sucrose). look at the bottom right at Est Final Gravity field. if you keep pushing the Increase Amt button and you will see the Est Final Gravity # go down as the amount of sugar increases
in my imperial pumpkin, instead of sugar for the day 6 addition, I used Lyle's Golden Syrup. took a gravity before I added it and it was 1.018. before I added the day 8 sugar, the gravity was 1.018.
final gravity reading going into secondary was 1.004, so that lb of sugar definitely dried it out.