If your wine is clear, rack off of the sediment and add your sugar. I'd start by tasting it dry and see what you think. If you don't like it dry---most people don't---then I'd start with your half cup per gallon. Then taste again, and add more if you think it needs it.
Like I've warned in the recipe, it will deepen in flavor as time goes by in the bottle, so be very careful how much sugar you add. Go to the point where you think it could use just a little bit more sugar and stop. Let it sit as prescribed for a week and taste again. You can always add more sugar, but you can't take it out.