That is correct. If that is confusing, try this:
Figure out the total gravity points you are adding. 1.042 = 42 points. So 2 lbs would be 84 points.
Divide the total additional points by the number of gallons. Say 1 lb in 2 gallons = 42/2 = 21. Then add this number to the original starting gravity; if 1.060, it would = 1.081.