If I understand your question, you don't need to calculate the sugars from the gravity...
corn sugar adds 46 points of gravity, per pound, per gallon. So to figure out how much sugar you need:
Lbs of sugar = [(desired grav - current grav) *1000*gallons] / 46
Ex: you have 5 gallons of 1.050, and want 1.060:
Lbs of sugar = [(1.060 - 1.050)*1000]* 5 / 46 = 1.1