Hmmm, BeerCalculus has Belgian Candi Sugar at around 35 gravity points per pound, which seems low but anyway using that value:
Take your OG - lets say 1.060 for example. That's 60 gravity points in 5 gallons, or 300 GPs. If you add a pound of candi sugar, you now have 335 GPs in the wort. divide by the 5 gallons, you get 1.066 as your effective OG (regardless of the fact that some of the fermentation has already happened).
Another (possibly simpler) way to think of it is that each pound of candi sugar will raise the gravity of a 5 gallon batch by .006.