It really depends on the carbonation level you would prefer. If you are looking to make the beer "correct for the style", then use John Palmer's recommendations. He knows far more about brewing than I do (and probably far more than whoever wrote your instruction sheet!)
With that being said, let me say that I prefer more than the "correct" amount of carbonation in my stouts. I usually use 4 to 5 ounces of corn sugar for most styles of beer. This is a personal preference, and I make the beer how I want it. If I entered a competition, I would probably lose points for the beer being "overcarbonated for the style", but that is not going to stop me from making it the way that I like it.
So it is really up to you. Anywhere between 2.8 ounces and 5 ounces will work, it just matters how carbonated you want the final beer to be...
"Give a man a beer, and he will waste an hour. Teach a man to brew, and he will waste a lifetime!" Bill Owen